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https://english-heritage.org.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
68
GRADE
D
FIX
6
REVIEW
2
PASS
7
INFO
2

Performance-focused optimization plan with CWV assessment, top offenders, and time estimates

Mobile Desktop
30
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
79
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
100
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
85
SEO Measures basic SEO optimizations: meta tags, crawlability, link text, and mobile friendliness.

First Contentful Paint First Contentful Paint — how long until the browser renders the first piece of content. Under 1.8s is good.

7.25 s

Largest Contentful Paint Largest Contentful Paint — how long until the largest visible element loads. Under 2.5s is good.

36.11 s

Total Blocking Time Total Blocking Time — total time the main thread was blocked, preventing user input. Under 200ms is good.

1.44 s

Cumulative Layout Shift Cumulative Layout Shift — measures visual stability. How much the page layout shifts during loading. Under 0.1 is good.

0.000

Speed Index Speed Index — how quickly content is visually displayed during load. Under 3.4s is good.

11.57 s

Time to Interactive Time to Interactive — how long until the page is fully interactive and responds to user input. Under 3.8s is good.

36.50 s

Page Load Progression

594 ms
1.2 s
1.8 s
2.4 s
3.0 s
3.6 s
4.2 s
4.8 s
Checks
17
7 PASS 2 REVIEW 6 FIX
F
Page Weight Budget
Action
6.3 MB transferred, 102 requests
FIX
6.3 MB transferred, 102 requests
Critical::
Page weighs 11.7 MB (6.3 MB transferred)
Warning::
Images are 3.7 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 3.7 MB
Warning::
JavaScript is 1.6 MB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 1.6 MB
Info::
Fonts are 500 KB — consider subsetting or using system fonts
Subset fonts to include only used characters, or switch to system font stacks.
Got: 500 KB
Info::
CSS is 251 KB — consider purging unused styles
Remove unused CSS rules with tools like PurgeCSS to reduce stylesheet size.
Got: 251 KB
Warning::
102 HTTP requests — consider bundling or reducing
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
Info::
Estimated 1.3 g CO2 per page load
6.3 MB / 2.4 MB budget
6.3 MB transferred
102 requests
1.3 g CO2 per page load
Images 3.7 MiB 60%
JavaScript 1.6 MiB 26%
Fonts 500.3 KiB 8%
CSS 251.5 KiB 4%
Other 108.9 KiB 2%
Other 41.9 KiB 1%
HTML 18.1 KiB 0%
Other 531 B 0%
Other 465 B 0%
Other 0 B 0%
0 2.4 MB 4.9 MB
Over budget

Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.

Why this matters

Image bandwidth is high — modern formats (WebP/AVIF) and resizing typically cut it 50%+.

Source: web.dev

Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.

Why this matters

JavaScript bundle is large — code-split routes and lazy-load off-screen components to defer.

Source: web.dev

Subset fonts to include only used characters, or switch to system font stacks.

Why this matters

Font payload is large — subset to used glyphs only, or fall back to system fonts (zero load time).

Source: web.dev font loading

Remove unused CSS rules with tools like PurgeCSS to reduce stylesheet size.

Why this matters

CSS bundle is large — purge unused styles (PurgeCSS, Tailwind JIT) to reduce CSS payload.

Source: web.dev

Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.

Why this matters

High HTTP request count — bundling, sprite-ing, and HTTP/2 server push can reduce per-request overhead.

Source: web.dev

D
Third-Party Impact
Action
100% third-party, 0 ms blocking
FIX
100% third-party, 0 ms blocking
Warning::
Third-party code accounts for 100% of page weight (6.3 MiB of 6.3 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
F
JS Execution Cost
Action
5835ms total JS execution
FIX
5835ms total JS execution
Warning::
https://www.english-heritage.org.uk/: 1630ms CPU time
Warning::
Unattributable: 640ms CPU time
Warning::
https://www.english-heritage.org.uk/bundles/js_pos...: 589ms CPU time
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-KF5...: 465ms CPU time
Warning::
https://cdn.checkout.ventrata.com/v3/production/ve...: 353ms CPU time
Warning::
Third-party scripts: 5835ms (100% of total)

Main Thread Breakdown

Script Evaluation 42% Other 27% Style & Layout 16% Rendering 6% Script Parsing & Compilation 5%
ScriptTotalScriptingParse/CompileParty
https://www.english-heritage.org.uk/1630ms24ms5ms3rd
Unattributable640ms11ms0ms3rd
https://www.english-heritage.org.uk/bundles/js_post_main_content?v=GYAGGeBAmVtYAWHOf2_28zxj7hsHoByHd_cHW7p0hRw1589ms130ms33ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-KF5D86465ms406ms47ms3rd
https://cdn.checkout.ventrata.com/v3/production/ventrata-checkout.min.js353ms161ms0ms3rd
https://www.googletagmanager.com/gtag/js?id=G-QK86RM1N34&cx=c&gtm=4e64k0337ms242ms38ms3rd
https://cdn.cookie-script.com/s/f4e7e1ee81fa8fb33e400f57fa93afb5.js304ms275ms12ms3rd
https://www.english-heritage.org.uk/bundles/js_pre_main_content?v=dZJA92ABTyjvwTaH4Viak-dmTT73fYBdBaxBwrJcK541241ms198ms25ms3rd
https://scripts.clarity.ms/0.8.60-beta/clarity.js196ms188ms5ms3rd
https://translate.googleapis.com/_/translate_http/_/js/k=translate_http.tr.es.eNjH1Jo6ldQ.O/am=AAAAAQ/d=1/exm=el_conf/ed=1/rs=AN8SPfq_osd-q7vwQm5aouAQRZibUmx4sg/m=el_main184ms125ms27ms3rd

Showing top 10 of 18 scripts

F
JS Bundles
Action
25 scripts, 770 KB unused
FIX
25 scripts, 770 KB unused
Warning::
https://cdn.checkout.ventrata.com/v3/production/as...: 122 KB unused (73%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.english-heritage.org.uk/bundles/js_pre...: 97 KB unused (73%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://translate.googleapis.com/_/translate_http/...: 93 KB unused (78%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.english-heritage.org.uk/bundles/js_pos...: 78 KB unused (73%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/destination?...: 67 KB unused (50%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 770 KB
770 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://cdn.checkout.ventrata.com/v3/production/assets/index-CVF5e-YI.js166 KB122 KB (73%)✓3rd
https://www.english-heritage.org.uk/bundles/js_pre_main_content?v=dZJA92ABTyjvwTaH4Viak-dmTT73fYBdBaxBwrJcK541132 KB97 KB (73%)✓3rd
https://translate.googleapis.com/_/translate_http/_/js/k=translate_http.tr.es.eNjH1Jo6ldQ.O/am=AAAAAQ/d=1/exm=el_conf/ed=1/rs=AN8SPfq_osd-q7vwQm5aouAQRZibUmx4sg/m=el_main119 KB93 KB (78%)✓3rd
https://www.english-heritage.org.uk/bundles/js_post_main_content?v=GYAGGeBAmVtYAWHOf2_28zxj7hsHoByHd_cHW7p0hRw1108 KB78 KB (73%)✓3rd
https://www.googletagmanager.com/gtag/destination?id=AW-859685842&cx=c&gtm=4e64k0134 KB67 KB (50%)✓3rd
https://www.googletagmanager.com/gtag/destination?id=AW-955260448&cx=c&gtm=4e64k0134 KB66 KB (50%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-QK86RM1N34&cx=c&gtm=4e64k0180 KB66 KB (37%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-10880701255&cx=c&gtm=4e64k0144 KB64 KB (45%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-KF5D86194 KB64 KB (33%)✓3rd
https://cdn.checkout.ventrata.com/v3/production/assets/prevention-ByXp18r8.js101 KB54 KB (53%)✓3rd

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

F
Page Weight Inventory
Action
102 resources · 6.3 MB · 770 KB savings available
FIX
102 resources · 6.3 MB · 770 KB savings available
Warning::
Page weight 5.3 MB over 1MB target
Got: 6.3 MB Expected: ≤ 1 MB
Info::
~770 KB of savings available
Warning::
101 third-party resources (100% of weight)
Info::
6 resources over 200KB
102 resources · 6.3 MiB · 101 third-party 5.3 MiB over 1MB target
Image 60% · 3.7 MiB
JavaScript 26% · 1.6 MiB
Font 8% · 500.3 KiB
CSS 4% · 251.5 KiB
Other 2% · 151.4 KiB
Document 0% · 18.2 KiB

~770.5 KiB of savings available

Remove unused JavaScript: 770.5 KiB
F
HTTP Caching
Action
No cache headers
FIX
No cache headers
Warning::
No Cache-Control header found
Browsers will use heuristic caching, which can be unpredictable. Set explicit cache headers.

No Cache-Control header

Adding a Cache-Control header can significantly improve repeat-visit performance.

B
Image Optimization
35 images, 0 KB saveable
REVIEW
35 images, 0 KB saveable
Warning::
https://www.english-heritage.org.uk/siteassets/hom... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
35 images 3.7 MB
0 oversized -0 KB
24 legacy format
1 missing dimensions CLS risk

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A+
Text Compression
All text resources are compressed
PASS
All text resources are compressed
Info::
All text resources are compressed
All text resources are properly compressed.
A+
Font Loading
13 fonts (500 KB)
PASS
13 fonts (500 KB)
Info::
13 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

13

500 KB total

Render-blocking

0

of 13

Dominant font-display

swap

Most common across fonts

Font loading timeline
Transfer FOIT (block) FOUT (swap)
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s 16.0 s 18.0 s 20.0 s 22.0 s 24.0 s 26.0 s 28.0 s 30.0 s 32.0 s 34.0 s 36.0 s FCP 7.3 s LCP 36.1 shttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.ttfGillSansW01Book.ttf https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.ttf — 50.9 KiB — 75 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.ttfGillSansW01Light.ttf https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.ttf — 44.0 KiB — 32 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.ttfGillSansW01Medium.ttf https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.ttf — 43.3 KiB — 76 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.ttfGillSansW01-LightItalic... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.ttf — 49.6 KiB — 76 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.ttfslick.ttf https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.ttf — 1.9 KiB — 76 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.woffGillSansW01Book.woff https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.woff — 26.9 KiB — 27 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.woffGillSansW01Light.woff https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.woff — 28.1 KiB — 77 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.woffGillSansW01Medium.woff https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.woff — 28.3 KiB — 78 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.woffGillSansW01-LightItalic... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.woff — 31.8 KiB — 79 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.woffslick.woff https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.woff — 1.4 KiB — 83 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Light.woffHeritageModerneWeb-Ligh... https://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Light.woff — 64.4 KiB — 33 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Medium.woffHeritageModerneWeb-Medi... https://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Medium.woff — 65.2 KiB — 37 ms — font-display: swaphttps://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Regular.woffHeritageModerneWeb-Regu... https://www.english-heritage.org.uk/static/scss/web-fonts/heritage-moderne/HeritageModerneWeb-Regular.woff — 64.7 KiB — 36 ms — font-display: swap
GillSansW01Book.ttf ttf swap
Size 51 KB
Load time 75 ms
Start 200 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01Light.ttf ttf swap
Size 44 KB
Load time 32 ms
Start 201 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01Medium.ttf ttf swap
Size 43 KB
Load time 76 ms
Start 201 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01-LightItalic... ttf swap
Size 50 KB
Load time 76 ms
Start 201 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
slick.ttf ttf swap
Size 2 KB
Load time 76 ms
Start 201 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01Book.woff woff swap
Size 27 KB
Load time 27 ms
Start 202 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01Light.woff woff swap
Size 28 KB
Load time 77 ms
Start 203 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01Medium.woff woff swap
Size 28 KB
Load time 78 ms
Start 203 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
GillSansW01-LightItalic... woff swap
Size 32 KB
Load time 79 ms
Start 203 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
slick.woff woff swap
Size 1 KB
Load time 83 ms
Start 204 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
HeritageModerneWeb-Ligh... woff swap
Size 64 KB
Load time 33 ms
Start 607 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
HeritageModerneWeb-Medi... woff swap
Size 65 KB
Load time 37 ms
Start 607 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
HeritageModerneWeb-Regu... woff swap
Size 65 KB
Load time 36 ms
Start 607 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
Optimization checklist
  • Preload critical fonts (priority=high)
  • Use woff2 format for all fonts
  • Set font-display to swap, optional, or fallback
  • Subset large fonts (≤100 KB each)

Fixing the unchecked items could save ~150 KB and ~300 ms

A+
Resource Caching
All resources properly cached
PASS
All resources properly cached
Info::
No caching issues found
All static resources have appropriate caching headers.
A+
Critical Rendering Path
No render-blocking resources
PASS
No render-blocking resources
Info::
No render-blocking resources detected
A+
Resource Hints
10 hints, 0 missing preconnects
PASS
10 hints, 0 missing preconnects
Info::
Page uses 10 resource hint(s)

Current Resource Hints

preconnect
—
preload
10
dns-prefetch
—
prefetch
—

10 resource hints configured

A+
Render-Blocking Resources
No render-blocking resources detected
PASS
No render-blocking resources detected
Info::
No render-blocking resources detected in <head>
A+
Third-Party Resources
No third-party resources detected
PASS
No third-party resources detected
Network Waterfall
102 requests over 3455ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s 16.0 s 18.0 s 20.0 s 22.0 s 24.0 s 26.0 s 28.0 s 30.0 s 32.0 s 34.0 s 36.0 s FCP 7.3 s LCP 36.1 shttps://english-heritage.org.uk// https://english-heritage.org.uk/ — 301 — 128 B — 23 ms — h2https://www.english-heritage.org.uk/www.english-heritage.org.uk/ https://www.english-heritage.org.uk/ — 200 — 16.6 KiB — 173 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.ttfwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.ttf — 200 — 50.9 KiB — 75 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.ttfwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.ttf — 200 — 44.0 KiB — 32 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.ttfwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.ttf — 200 — 43.3 KiB — 76 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.ttfwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.ttf — 200 — 49.6 KiB — 76 ms — h2https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.ttfwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.ttf — 200 — 1.9 KiB — 76 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.woffwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Book.woff — 200 — 26.9 KiB — 27 ms — h2https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.woffwww.english-heritage.org.uk/static/... 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https://www.english-heritage.org.uk/static/scss/appcss?v=XMttbVtDm4ZzSEUOS1L1C9CIRFBpDkKInvkFyQE0yJg1 — 200 — 236.0 KiB — 68 ms — h2https://www.english-heritage.org.uk/bundles/js_pre_main_content?v=dZJA92ABTyjvwTaH4Viak-dmTT73fYBdBaxBwrJcK541www.english-heritage.org.uk/bundles... https://www.english-heritage.org.uk/bundles/js_pre_main_content?v=dZJA92ABTyjvwTaH4Viak-dmTT73fYBdBaxBwrJcK541 — 200 — 132.5 KiB — 84 ms — h2https://www.english-heritage.org.uk/siteassets/home/brand2024/spring-2026/header-spring-2-1920x1080.jpgwww.english-heritage.org.uk/siteass... https://www.english-heritage.org.uk/siteassets/home/brand2024/spring-2026/header-spring-2-1920x1080.jpg — 200 — 194.9 KiB — 92 ms — h2https://www.english-heritage.org.uk/siteassets/home/brand2024/spring-2026/may-homepage-header-1-1920x1080.jpgwww.english-heritage.org.uk/siteass... https://www.english-heritage.org.uk/siteassets/home/brand2024/spring-2026/may-homepage-header-1-1920x1080.jpg — 200 — 793.7 KiB — 109 ms — 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https://pagead2.googlesyndication.com/ccm/collect?rcb=13&frm=0&ae=g&dl=https%3A%2F%2Fwww.english-heritage.org.uk%2F&scrsrc=www.googletagmanager.com&rnd=1993784578.1776892076&navt=n&npa=1&gdpr=1&gdpr_consent=tcempty&epn.LandingYear=2026&ep.LandingMonth=April&epn.SessionCount=1&gdid=dMmY1Mm&_tu=CA&gtm=45be64k0z86888636za20gzb6888636zd6888636xea&gcs=G100&gcd=13q3p3m2m5l1&dma_cps=-&dma=1&tcfd=15S5b&tag_exp=0~115616985~115938465~115938468~117266400~117384406&apve=1&apvf=f&apvc=0&tids=AW-955260448&tid=AW-955260448&tft=1776892076220&tfd=1441 — 200 — 21 B — 33 ms — h3https://pagead2.googlesyndication.com/ccm/collect?rcb=16&frm=0&ae=g&dl=https%3A%2F%2Fwww.english-heritage.org.uk%2F&scrsrc=www.googletagmanager.com&rnd=1993784578.1776892076&navt=n&npa=1&gdpr=1&gdpr_consent=tcempty&gdid=dMmY1Mm&_tu=CA&gtm=45be64k0z86888636za20gzb6888636zd6888636xea&gcs=G100&gcd=13q3p3m2m5l1&dma_cps=-&dma=1&tcfd=15S5b&tag_exp=0~115938465~115938469~117266400&apve=1&apvf=f&apvc=0&tids=AW-859685842&tid=AW-859685842&tft=1776892076221&tfd=1442pagead2.googlesyndication.com/ccm/c... https://pagead2.googlesyndication.com/ccm/collect?rcb=16&frm=0&ae=g&dl=https%3A%2F%2Fwww.english-heritage.org.uk%2F&scrsrc=www.googletagmanager.com&rnd=1993784578.1776892076&navt=n&npa=1&gdpr=1&gdpr_consent=tcempty&gdid=dMmY1Mm&_tu=CA&gtm=45be64k0z86888636za20gzb6888636zd6888636xea&gcs=G100&gcd=13q3p3m2m5l1&dma_cps=-&dma=1&tcfd=15S5b&tag_exp=0~115938465~115938469~117266400&apve=1&apvf=f&apvc=0&tids=AW-859685842&tid=AW-859685842&tft=1776892076221&tfd=1442 — 200 — 21 B — 35 ms — h3https://bat.bing.net/action/0?ti=271007761&tm=gtm002&Ver=2&mid=06a73a19-cea4-4539-875d-093c6a3beb74&bo=3&pi=918639831&lg=en-US&sw=412&sh=823&sc=24&tl=English%20Heritage%20%7C%20English%20Heritage&p=https%3A%2F%2Fwww.english-heritage.org.uk%2F&r=&lt=837&mtp=1&evt=pageLoad&sv=2&asc=D&cdb=AQkV&rn=476365bat.bing.net/action/0?ti=271007761&... https://bat.bing.net/action/0?ti=271007761&tm=gtm002&Ver=2&mid=06a73a19-cea4-4539-875d-093c6a3beb74&bo=3&pi=918639831&lg=en-US&sw=412&sh=823&sc=24&tl=English%20Heritage%20%7C%20English%20Heritage&p=https%3A%2F%2Fwww.english-heritage.org.uk%2F&r=&lt=837&mtp=1&evt=pageLoad&sv=2&asc=D&cdb=AQkV&rn=476365 — 204 — 122 B — 47 ms — h2https://consent.cookie-script.com/analytics?action=firstshown&time=1776892076328&script=f4e7e1ee81fa8fb33e400f57fa93afb5&category=consent.cookie-script.com/analytics... https://consent.cookie-script.com/analytics?action=firstshown&time=1776892076328&script=f4e7e1ee81fa8fb33e400f57fa93afb5&category= — 200 — 293 B — 456 ms — h2https://dc.services.visualstudio.com/v2/trackdc.services.visualstudio.com/v2/tra... https://dc.services.visualstudio.com/v2/track — 204 — 0 B — 241 ms — h2https://www.english-heritage.org.uk/favicon.icowww.english-heritage.org.uk/favicon... https://www.english-heritage.org.uk/favicon.ico — 200 — 337 B — 31 ms — h3https://j.clarity.ms/collectj.clarity.ms/collect https://j.clarity.ms/collect — 204 — 291 B — 573 ms — http/1.1https://dc.services.visualstudio.com/v2/trackdc.services.visualstudio.com/v2/tra... https://dc.services.visualstudio.com/v2/track — 200 — 200 B — 113 ms — h2https://j.clarity.ms/collectj.clarity.ms/collect https://j.clarity.ms/collect — 204 — 291 B — 101 ms — http/1.1
HTML JavaScript CSS Images Fonts XHR/Fetch Other
Third-Party Script Cost
Per-script blocking time, transfer cost, and cache headers
INFO
100% of JavaScript execution is third-party
First-party Third-party 5835ms · 1739KB · £30/mo
www.english-heritage.org.uk
Other
Costly
Execution 1630ms
Transfer 17 KB
Monthly Cost £8/mo
Unattributable
Other
Costly
Execution 640ms
Transfer 0 KB
Monthly Cost £3/mo
www.english-heritage.org.uk
Other
Costly
Execution 589ms
Transfer 108 KB
Unused 73%
Monthly Cost £3/mo
Google Tag Manager
Tag Manager
Costly
Execution 465ms
Transfer 194 KB
Unused 33%
Monthly Cost £2/mo
cdn.checkout.ventrata.com
Other
Costly
Execution 353ms
Transfer 31 KB
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Costly
Execution 337ms
Transfer 180 KB
Unused 37%
Monthly Cost £2/mo
Cookie Script
Consent
Essential
Execution 304ms
Transfer 31 KB
Monthly Cost £2/mo
www.english-heritage.org.uk
Other
Costly
Execution 241ms
Transfer 132 KB
Unused 73%
Monthly Cost £1/mo
Microsoft Clarity
Session Replay
Optional
Execution 196ms
Transfer 25 KB
Monthly Cost £1/mo
translate.googleapis.com
Other
Optional
Execution 184ms
Transfer 119 KB
Unused 78%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 168ms
Transfer 144 KB
Unused 45%
Monthly Cost £1/mo
Bing UET
Advertising
Optional
Execution 166ms
Transfer 15 KB
Monthly Cost £1/mo
cdn.checkout.ventrata.com
Other
Optional
Execution 163ms
Transfer 166 KB
Unused 73%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 112ms
Transfer 134 KB
Unused 50%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 99ms
Transfer 134 KB
Unused 50%
Monthly Cost £1/mo
www.english-heritage.org.uk
Other
Optional
Execution 76ms
Transfer 236 KB
Monthly Cost £0/mo
js.monitor.azure.com
Other
Optional
Execution 59ms
Transfer 22 KB
Monthly Cost £0/mo
www.english-heritage.org.uk
Other
Optional
Execution 52ms
Transfer 52 KB
Monthly Cost £0/mo

These scripts may cost more than they're worth

  • www.english-heritage.org.uk adds 1630ms and costs ~£8/month
  • Unattributable adds 640ms and costs ~£3/month
  • www.english-heritage.org.uk adds 589ms and costs ~£3/month
  • Google Tag Manager adds 465ms and costs ~£2/month
  • cdn.checkout.ventrata.com adds 353ms and costs ~£2/month
  • Google Tag Manager adds 337ms and costs ~£2/month
  • www.english-heritage.org.uk adds 241ms and costs ~£1/month

100% of JavaScript execution time is spent on third-party scripts. Consider auditing which scripts are essential.

Why this matters

When third-party JS execution time exceeds your own, performance gains from frontend work are capped by code you don't own.

Learn more ▾

Every millisecond of third-party JS competes with your own for main-thread time. If their share is bigger than yours, optimization on your code base barely moves the needle. Audit, defer, or remove third-parties before further frontend optimization. Use Lighthouse's third-party audit to identify the worst offenders.

Source: web.dev

www.english-heritage.org.uk takes 1630ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

Unattributable takes 640ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

www.english-heritage.org.uk takes 589ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

73% of www.english-heritage.org.uk's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

Source: web.dev

Google Tag Manager takes 465ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

cdn.checkout.ventrata.com takes 353ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

Google Tag Manager takes 337ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

www.english-heritage.org.uk takes 241ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

73% of www.english-heritage.org.uk's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

Source: web.dev

78% of translate.googleapis.com's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

Source: web.dev

73% of cdn.checkout.ventrata.com's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

Source: web.dev

50% of Google Tag Manager's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

Source: web.dev

How third-party costs are calculated

All checks on this page are automated. Results are estimates - run targeted manual reviews when the score affects a release decision.

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