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https://english-heritage.org.uk

Performance

· 15 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
68
GRADE
D
FIX
5
REVIEW
3
PASS
5
INFO
2

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

Mobile Desktop
70
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
82
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.

1.21 s

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

4.17 s

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

28 ms

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

0.004

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

2.62 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.

5.42 s

Page Load Progression

571 ms
1.1 s
1.7 s
2.3 s
2.9 s
3.4 s
4.0 s
4.6 s
Checks
15
5 PASS 3 REVIEW 5 FIX
F
Page Weight Budget
Action
6.3 MB transferred, 100 requests
FIX
6.3 MB transferred, 100 requests
Critical::
Page weighs 11.8 MB (6.3 MB transferred)
Warning::
Images are 3.8 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 3.8 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 502 KB — consider subsetting or using system fonts
Subset fonts to include only used characters, or switch to system font stacks.
Got: 502 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
Info::
100 HTTP requests
Info::
Estimated 1.3 g CO2 per page load
6.3 MB / 2.4 MB budget
6.3 MB transferred
100 requests
1.3 g CO2 per page load
Images 3.8 MiB 60%
JavaScript 1.6 MiB 26%
Fonts 501.6 KiB 8%
CSS 251.4 KiB 4%
Other 109.0 KiB 2%
Other 41.9 KiB 1%
HTML 18.0 KiB 0%
Other 1.4 KiB 0%
Other 466 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

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 Bundles
Action
25 scripts, 772 KB unused
FIX
25 scripts, 772 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...: 96 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: 772 KB
772 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 KB96 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 KB67 KB (50%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-QK86RM1N34&cx=c&gtm=4e64k0180 KB67 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
100 resources · 6.3 MB · 772 KB savings available
FIX
100 resources · 6.3 MB · 772 KB savings available
Warning::
Page weight 5.3 MB over 1MB target
Got: 6.3 MB Expected: ≤ 1 MB
Info::
~772 KB of savings available
Warning::
99 third-party resources (100% of weight)
Info::
6 resources over 200KB
100 resources · 6.3 MiB · 99 third-party 5.3 MiB over 1MB target
Image 60% · 3.8 MiB
JavaScript 26% · 1.6 MiB
Font 8% · 501.6 KiB
CSS 4% · 251.4 KiB
Other 2% · 151.4 KiB
Document 0% · 18.9 KiB

~771.8 KiB of savings available

Remove unused JavaScript: 771.8 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
33 images, 0 KB saveable
REVIEW
33 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.
33 images 3.8 MB
0 oversized -0 KB
23 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
JS Execution Cost
Action
1324ms total JS execution
REVIEW
1324ms total JS execution
Warning::
https://www.english-heritage.org.uk/: 459ms CPU time
Info::
Unattributable: 191ms CPU time
Info::
https://cdn.checkout.ventrata.com/v3/production/ve...: 186ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-KF5...: 130ms CPU time
Info::
https://cdn.cookie-script.com/s/f4e7e1ee81fa8fb33e...: 87ms CPU time
Warning::
Third-party scripts: 1324ms (100% of total)

Main Thread Breakdown

Script Evaluation 46% Other 27% Style & Layout 14% Script Parsing & Compilation 5% Rendering 5%
ScriptTotalScriptingParse/CompileParty
https://www.english-heritage.org.uk/459ms7ms1ms3rd
Unattributable191ms6ms0ms3rd
https://cdn.checkout.ventrata.com/v3/production/ventrata-checkout.min.js186ms137ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-KF5D86130ms115ms12ms3rd
https://cdn.cookie-script.com/s/f4e7e1ee81fa8fb33e400f57fa93afb5.js87ms81ms3ms3rd
https://www.googletagmanager.com/gtag/js?id=G-QK86RM1N34&cx=c&gtm=4e64k079ms68ms10ms3rd
https://www.english-heritage.org.uk/bundles/js_pre_main_content?v=dZJA92ABTyjvwTaH4Viak-dmTT73fYBdBaxBwrJcK54174ms55ms10ms3rd
https://www.english-heritage.org.uk/bundles/js_post_main_content?v=GYAGGeBAmVtYAWHOf2_28zxj7hsHoByHd_cHW7p0hRw167ms35ms5ms3rd
https://bat.bing.com/bat.js51ms38ms1ms3rd
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 (502 KB)
PASS
13 fonts (502 KB)
Info::
13 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

13

502 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 1.0 s 2.0 s 3.0 s 4.0 s FCP 1.2 s LCP 4.2 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 — 51.1 KiB — 99 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 — 44.1 KiB — 100 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 — 43.4 KiB — 101 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 — 49.7 KiB — 103 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 — 2.1 KiB — 102 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 — 115 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 — 116 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 — 28.4 KiB — 118 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 — 31.9 KiB — 118 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 — 1.6 KiB — 117 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 — 42 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 — 41 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 — 31 ms — font-display: swap
GillSansW01Book.ttf ttf swap
Size 51 KB
Load time 99 ms
Start 178 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 100 ms
Start 178 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 101 ms
Start 179 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 103 ms
Start 179 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 102 ms
Start 179 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 115 ms
Start 180 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 116 ms
Start 180 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 118 ms
Start 180 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 118 ms
Start 181 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
slick.woff woff swap
Size 2 KB
Load time 117 ms
Start 181 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 42 ms
Start 401 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 41 ms
Start 401 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 31 ms
Start 535 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
No optimization needed
PASS
No optimization needed
Info::
No resource hint issues
Network Waterfall
100 requests over 2260ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s FCP 1.2 s LCP 4.2 shttps://english-heritage.org.uk// https://english-heritage.org.uk/ — 301 — 128 B — 19 ms — h2https://www.english-heritage.org.uk/www.english-heritage.org.uk/ https://www.english-heritage.org.uk/ — 200 — 16.6 KiB — 168 ms — h3https://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 — 51.1 KiB — 99 ms — h3https://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 — 44.1 KiB — 100 ms — h3https://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 — 43.4 KiB — 101 ms — h3https://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 — 49.7 KiB — 103 ms — h3https://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 — 2.1 KiB — 102 ms — h3https://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 — 115 ms — h3https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.woffwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Light.woff — 200 — 28.1 KiB — 116 ms — h3https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.woffwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01Medium.woff — 200 — 28.4 KiB — 118 ms — h3https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.woffwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/web-fonts/GillSansW01-LightItalic.woff — 200 — 31.9 KiB — 118 ms — h3https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.woffwww.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scripts/libs/slick/fonts/slick/slick.woff — 200 — 1.6 KiB — 117 ms — h3https://www.english-heritage.org.uk/static/scss/appcss?v=XMttbVtDm4ZzSEUOS1L1C9CIRFBpDkKInvkFyQE0yJg1www.english-heritage.org.uk/static/... https://www.english-heritage.org.uk/static/scss/appcss?v=XMttbVtDm4ZzSEUOS1L1C9CIRFBpDkKInvkFyQE0yJg1 — 200 — 236.0 KiB — 96 ms — h3https://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 — 121 ms — h3https://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 — 194.7 KiB — 147 ms — h3https://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 — 793.1 KiB — 142 ms — h3https://cdn.checkout.ventrata.com/v3/production/ventrata-checkout.min.jscdn.checkout.ventrata.com/v3/produc... 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https://pagead2.googlesyndication.com/ccm/collect?rcb=14&frm=0&ae=g&dl=https%3A%2F%2Fwww.english-heritage.org.uk%2F&scrsrc=www.googletagmanager.com&rnd=1400506241.1776892101&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=0~115938465~115938469~117266401&apve=1&apvf=f&apvc=0&tids=AW-955260448&tid=AW-955260448&tft=1776892101945&tfd=1536 — 200 — 21 B — 78 ms — h3https://bat.bing.net/action/0?ti=271007761&tm=gtm002&Ver=2&mid=904c7f44-b70e-46bb-8613-fe135b5fc468&bo=3&pi=918639831&lg=en-US&sw=800&sh=600&sc=24&tl=English%20Heritage%20%7C%20English%20Heritage&p=https%3A%2F%2Fwww.english-heritage.org.uk%2F&r=&lt=886&evt=pageLoad&sv=2&asc=D&cdb=AQkV&rn=330506bat.bing.net/action/0?ti=271007761&... https://bat.bing.net/action/0?ti=271007761&tm=gtm002&Ver=2&mid=904c7f44-b70e-46bb-8613-fe135b5fc468&bo=3&pi=918639831&lg=en-US&sw=800&sh=600&sc=24&tl=English%20Heritage%20%7C%20English%20Heritage&p=https%3A%2F%2Fwww.english-heritage.org.uk%2F&r=&lt=886&evt=pageLoad&sv=2&asc=D&cdb=AQkV&rn=330506 — 204 — 121 B — 47 ms — h2https://consent.cookie-script.com/analytics?action=firstshown&time=1776892102034&script=f4e7e1ee81fa8fb33e400f57fa93afb5&category=consent.cookie-script.com/analytics... https://consent.cookie-script.com/analytics?action=firstshown&time=1776892102034&script=f4e7e1ee81fa8fb33e400f57fa93afb5&category= — 200 — 293 B — 419 ms — h2https://dc.services.visualstudio.com/v2/trackdc.services.visualstudio.com/v2/tra... https://dc.services.visualstudio.com/v2/track — 204 — 0 B — 279 ms — h2https://www.english-heritage.org.uk/favicon.icowww.english-heritage.org.uk/favicon... https://www.english-heritage.org.uk/favicon.ico — 200 — 336 B — 29 ms — h3https://j.clarity.ms/collectj.clarity.ms/collect https://j.clarity.ms/collect — 204 — 291 B — 548 ms — http/1.1https://j.clarity.ms/collectj.clarity.ms/collect https://j.clarity.ms/collect — 204 — 291 B — 343 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 — h2
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 1324ms · 708KB · £7/mo
www.english-heritage.org.uk
Other
Costly
Execution 459ms
Transfer 17 KB
Monthly Cost £2/mo
Unattributable
Other
Optional
Execution 191ms
Transfer 0 KB
Monthly Cost £1/mo
cdn.checkout.ventrata.com
Other
Optional
Execution 186ms
Transfer 31 KB
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 130ms
Transfer 194 KB
Unused 33%
Monthly Cost £1/mo
Cookie Script
Consent
Essential
Execution 87ms
Transfer 31 KB
Monthly Cost £0/mo
Google Tag Manager
Tag Manager
Optional
Execution 79ms
Transfer 180 KB
Unused 37%
Monthly Cost £0/mo
www.english-heritage.org.uk
Other
Optional
Execution 74ms
Transfer 132 KB
Unused 73%
Monthly Cost £0/mo
www.english-heritage.org.uk
Other
Optional
Execution 67ms
Transfer 108 KB
Unused 73%
Monthly Cost £0/mo
Bing UET
Advertising
Optional
Execution 51ms
Transfer 15 KB
Monthly Cost £0/mo

These scripts may cost more than they're worth

  • www.english-heritage.org.uk adds 459ms and costs ~£2/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 459ms 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

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

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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