Performance
· 28 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.Performance-focused optimization plan with CWV assessment, top offenders, and time estimates
First Contentful Paint First Contentful Paint — how long until the browser renders the first piece of content. Under 1.8s is good.
3.62 s
Largest Contentful Paint Largest Contentful Paint — how long until the largest visible element loads. Under 2.5s is good.
16.80 s
Total Blocking Time Total Blocking Time — total time the main thread was blocked, preventing user input. Under 200ms is good.
3.05 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.
6.96 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.
17.29 s
Page Load Progression
DPage Weight BudgetAction2.3 MB transferred, 128 requestsFIX
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
JavaScript bundle is large — code-split routes and lazy-load off-screen components to defer.
Source: web.dev
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
High HTTP request count — bundling, sprite-ing, and HTTP/2 server push can reduce per-request overhead.
Source: web.dev
DThird-Party ImpactAction98% third-party, 0 ms blockingFIX
FJS Execution CostAction6808ms total JS executionFIX
Main Thread Breakdown
| Script | Total | Scripting | Parse/Compile | Party |
|---|---|---|---|---|
| https://c77.bookbot.com/_next/static/chunks/framework-c6c82aad00023883.js | 2977ms | 1540ms | 12ms | 3rd |
| https://knihobot.cz/ | 909ms | 8ms | 1ms | 1st |
| Unattributable | 749ms | 23ms | 0ms | 1st |
| https://www.googletagmanager.com/gtm.js?id=GTM-TF5RV29 | 431ms | 376ms | 48ms | 3rd |
| https://c77.bookbot.com/_next/static/chunks/pages/_app-3f66b878bb9e3ed3.js | 369ms | 154ms | 38ms | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-N116K2C2SM&cx=c>m=4e6921 | 334ms | 285ms | 45ms | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-000&cx=c>m=4e6921 | 199ms | 157ms | 40ms | 3rd |
| https://c77.bookbot.com/_next/static/chunks/pages/index-c9875874d13834ca.js | 196ms | 188ms | 3ms | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=AW-894320112&cx=c>m=4e6921 | 187ms | 156ms | 30ms | 3rd |
| https://c77.bookbot.com/_next/static/chunks/main-9b896f863166457a.js | 113ms | 87ms | 21ms | 3rd |
Showing top 10 of 15 scripts
FJS BundlesAction65 scripts, 1.3 MB unusedFIX
Downloaded but never executed on this page.
| Script | Size | Unused | Minified | Party |
|---|---|---|---|---|
| https://c77.bookbot.com/_next/static/chunks/5581-5141ebacfaf076b5.js | 52 KB | 138 KB (73%) | ✓ | 3rd |
| https://c77.bookbot.com/_next/static/chunks/5581-5141ebacfaf076b5.js | 0 KB | 138 KB (73%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=AW-894320112&cx=c>m=4e6921 | 158 KB | 75 KB (47%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-N116K2C2SM&cx=c>m=4e6921 | 182 KB | 72 KB (40%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-000&cx=c>m=4e6921 | 147 KB | 69 KB (47%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtm.js?id=GTM-TF5RV29 | 196 KB | 65 KB (33%) | ✓ | 3rd |
| https://c77.bookbot.com/_next/static/chunks/4586-e8ea1f603aeb7f0a.js | 0 KB | 63 KB (56%) | ✓ | 3rd |
| https://c77.bookbot.com/_next/static/chunks/4586-e8ea1f603aeb7f0a.js | 24 KB | 63 KB (56%) | ✓ | 3rd |
| https://c77.bookbot.com/_next/static/chunks/pages/_app-3f66b878bb9e3ed3.js | 144 KB | 62 KB (43%) | ✓ | 3rd |
| https://c77.bookbot.com/_next/static/chunks/pages/sell-books-b3e6559af12f4c3c.js | 23 KB | 57 KB (73%) | ✓ | 3rd |
Consider code splitting or tree shaking to reduce unused code.
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.
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.
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.
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.
This bundle has high unused code — code-split or tree-shake to ship only what executes.
Source: web.dev / Lighthouse coverage
FPage Weight InventoryAction128 resources · 2.3 MB · 1.3 MB savings availableFIX
~1.3 MiB of savings available
| # | Resource | Type | Size | Cache | Unused | 3rd party | Issues |
|---|---|---|---|---|---|---|---|
| — | — | ||||||
| 128 resources | 2.3 MiB | ||||||
DHTTP CachingActionno-store (uncacheable)FIX
private, no-cache, no-store, max-age=0, must-revalidate
| Directive | Value | Meaning |
|---|---|---|
| private | — | Only browser cache, not CDN/proxy |
| no-cache | — | Cache but always revalidate first |
| no-store | — | Never cache this response |
| max-age | 0 | Cache for 0 seconds |
| must-revalidate | — | Must recheck with server after expiry |
BImage Optimization17 images, 0 KB saveableREVIEW
BHTTP/3 (QUIC)HTTP/3 not advertisedREVIEW
BDocument CompressionDocument response uses gzip; brotli would be ~20% smallerREVIEW
BServer-Timing ObservabilityNo Server-Timing header foundREVIEW
CGreen HostingActionWhether the site is served from green-energy infrastructureREVIEW
Green Hosting
No green hosting detected
A+Text CompressionAll text resources are compressedPASS
A+Font Loading4 fonts (105 KB)PASS
Web fonts
4
105 KB total
Render-blocking
0
of 4
Dominant font-display
swap
Most common across fonts
Font loading timeline
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)
A+Resource CachingAll resources properly cachedPASS
A+Critical Rendering PathNo render-blocking resourcesPASS
AResource Hints43 hints, 0 missing preconnectsPASS
Current Resource Hints
43 resource hints configured
Each preconnect eagerly opens a TCP+TLS connection. On slow networks or low-end devices, too many parallel connections cause head-of-line blocking and starve the connections that matter for LCP. Trim to the 4-6 highest-value origins (the ones serving render-blocking or LCP resources).
Informational: a labeled value pair from the audit.
The same href shouldn't appear in multiple hint rels -- one wins, the others are wasted markup and may confuse cache behavior. Sample: https://rezised-images.knhbt.cz (preconnect+preconnect+preconnect+preconnect+preconnect+preconnect+preconnect+preconnect+preconnect).
Performance issues directly impact user engagement and conversion rates.
AAsset Compression1 of 2 asset hosts still on gzipPASS
A+LCP Image PreloadLCP preload audit not availablePASS
A+Main HTML Cache-ControlMain HTML uses no-store -- correct for sensitive/dynamic contentPASS
A+Server Response Intelligence2 server-response signal(s) detectedPASS
A+Render-Blocking ResourcesNo render-blocking resources detectedPASS
A+Third-Party ResourcesNo third-party resources detectedPASS
AJavaScript Blocking1 JS blocking issue(s) detectedPASS
ACSS Performance Depth1 CSS depth issue(s) detectedPASS
Network Waterfall128 requests over 2997msINFO
Third-Party Script CostPer-script blocking time, transfer cost, and cache headersINFO
| Script | Category | Execution | Transfer | Unused | Monthly Cost | Verdict |
|---|---|---|---|---|---|---|
c77.bookbot.com c77.bookbot.com | Other | 2977ms | 45 KB | — | Kč1196/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 431ms | 196 KB | 33% | Kč173/mo | Costly |
c77.bookbot.com c77.bookbot.com | Other | 369ms | 144 KB | 43% | Kč148/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 334ms | 182 KB | 40% | Kč134/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 199ms | 147 KB | 47% | Kč80/mo | Costly |
c77.bookbot.com c77.bookbot.com | Other | 196ms | 8 KB | — | Kč79/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 187ms | 158 KB | 47% | Kč75/mo | Costly |
c77.bookbot.com c77.bookbot.com | Other | 113ms | 38 KB | — | Kč45/mo | Optional |
l.seznam.cz l.seznam.cz | Other | 81ms | 69 KB | 57% | Kč33/mo | Optional |
c77.bookbot.com c77.bookbot.com | Other | 73ms | 0 KB | 73% | Kč29/mo | Optional |
l.seznam.cz l.seznam.cz | Other | 72ms | 103 KB | 42% | Kč29/mo | Optional |
c77.bookbot.com c77.bookbot.com | Other | 61ms | 0 KB | 73% | Kč25/mo | Optional |
c77.bookbot.com c77.bookbot.com | Other | 58ms | 40 KB | 58% | Kč23/mo | Optional |
These scripts may cost more than they're worth
- c77.bookbot.com adds 2977ms and costs ~Kč1196/month
- Google Tag Manager adds 431ms and costs ~Kč173/month
- c77.bookbot.com adds 369ms and costs ~Kč148/month
- Google Tag Manager adds 334ms and costs ~Kč134/month
- Google Tag Manager adds 199ms and costs ~Kč80/month
- c77.bookbot.com adds 196ms and costs ~Kč79/month
- Google Tag Manager adds 187ms and costs ~Kč75/month
76% of JavaScript execution time is spent on third-party scripts. Consider auditing which scripts are essential.
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
c77.bookbot.com takes 2977ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 431ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
c77.bookbot.com takes 369ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 334ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 199ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
c77.bookbot.com takes 196ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 187ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
57% of l.seznam.cz's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
73% of c77.bookbot.com's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
73% of c77.bookbot.com's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
58% of c77.bookbot.com's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
Real-User Performance (CrUX)Field metrics from the Chrome User Experience ReportINFO
Chrome User Experience (CrUX)
Real-user metrics from the Chrome UX Report (2026-08-10 — 2026-09-06)
LCP (p75)
1841 ms
87% good
INP (p75)
463 ms
38% good
CLS (p75)
0.100
74% good
FCP (p75)
1723 ms
TTFB (p75)
760 ms
Image Optimization24 images, 240 KB total, 109 KB wastedINFO
Image Optimization Audit
Total Images
24
Total Size
240 KB
Oversized
4
Wasted
109 KB