PerfReviews
@perf.reviews
🇺🇸 Experts in web performance, we help you to solve performance issues 🇪🇸 Expertos en web performance, te ayudamos a solucionar problemas de rendimiento https://perf.reviews/
How far ahead browsers fetch lazy images: • Chrome: 1,250px on 4G, 2,500px on 3G, 6,000px (!!) on 2G • Firefox: 600px, on any connection • Safari: one viewport. Lazy iframes: 0px, they wait until on screen
Lazy Loading Images and Media: How Far Ahead Browsers Fetch
How far ahead Chrome, Firefox and Safari fetch lazy media, why a lazy hero image slows your page, and where to use lazy, eager and fetchpriority.
calibreapp.com
So happy to share mine & @nicj.net's first @cloudflare.social blog post—during our 16th Birthday Week no less! 🥳 Today we announce BEACON, a #webperf research dataset we're contributing to the @rumarchive.com community. blog.cloudflare.com/how-fast-is-...
How fast is the web? Explore billions of real-user measurements with BEACON
Cloudflare is open-sourcing the BEACON dataset, making billions of anonymized Real User Monitoring (RUM) performance records publicly available on Google BigQuery.
blog.cloudflare.com
The case against JPEG XL Investigating JPEG XL's place as a Web image codec, by @giannirosato.com, via @webperfreport.bsky.social giannirosato.com/blog/post/ca... #JPEGXL #JXL #ImagePerf #WebPerf
P75 is green: LCP 1.58s, INP 176ms, CLS 0.01. P90: LCP 2.68s, INP 352ms, CLS 0.17. 15% of visits get a noticeably worse experience. Product Performance doesn't end once you pass CrUX: if P75 is good, work on P90. #WebPerf #CoreWebVitals #RUM #ProductPerformance
🤩 Fantastic article by @nucliweb.net on HTTP cache partitioning and the #CrossOriginStorage API: joanleon.dev/en/http-cach.... Try the embedded demos with the COS extension installed or in one of the experimental browser builds (see the Awesome Cross-Origin Storage list: github.com/tomayac/awes...).
HTTP cache partitioning: the same image, downloaded twice | Joan León
A page with a cross-site iframe requests the same image from the iframe and from the parent. Chrome downloads it twice; Firefox and Safari, once. Why the cache key differs across the three engines, wh...
joanleon.dev
If you use @speedcurve.com—and you should—then LUX Sidecar is for you! csswizardry.com/2026/08/intr...
Introducing LUX Sidecar: Your SpeedCurve Companion – CSS Wizardry
LUX Sidecar is a small SpeedCurve RUM plugin that adds browser and navigation context to your real-user performance data.
csswizardry.com
“The performance issues erode trust. Users rarely report performance problems directly; they simply leave, convert less often, or switch to competitors.” - @denodell.bsky.social www.manning.com/books/fast-b... #WebPerf #FastByDefault #PerfEngineering
Fast by Default - Den Odell
You can prevent most software performance problems if you recognize their predictable patterns. Fast by Default: Practical performance engineering introduces author Den Odell’s Fast by Default model t...
manning.com
In my audits, the question I hear most after a deploy is "why hasn't CrUX moved yet?" The window is always 28 days; daily, weekly and monthly are just cadences over the same data. And p75 resists change by design I explain it with two interactive demos joanleon.dev/en/crux-28-d... #WebPerf #CrUX
The CrUX 28-day rolling window | Joan León
How the CrUX 28-day rolling window actually works, what each source publishes, and why p75 takes so long to move even though the data is only two days old.
joanleon.dev
Most skills are a Markdown file that grows out of control and gets loaded whole, even when the task doesn't need it. Modern Web Guidance inverts the pattern: 250 fixed tokens, and the agent retrieves the guide it needs on demand. joanleon.dev/en/modern-we... #AI #WebDev #CodingAgents
A skill that knows nothing: the architecture of Modern Web Guidance | Joan León
Modern Web Guidance doesn't keep the knowledge in the skill, it teaches the agent to retrieve it. Local semantic search with MiniLM and TensorFlow.js, guides with their own grader, and empirical calib...
joanleon.dev
Reading a Performance Timeline by @nucliweb.net. This would be great for people who still have issues with Performance Timeline in Chrome. I love the form of this site. perf.reviews/profile-guide
Reading a Performance Profile: Field Guide
An interactive field guide to the Chrome DevTools Performance panel: waterfall, flame chart, LCP, CLS, INP, and the Insights panel.
perf.reviews
This is an excellent explanation of the lag that exists in CrUX data Recently had a customer who made huge improvements to INP but it took nearly four weeks for it to show in CrUX
Joan León@nucliweb.net · 2mo agoIn my audits, the question I hear most after a deploy is "why hasn't CrUX moved yet?" The window is always 28 days; daily, weekly and monthly are just cadences over the same data. And p75 resists change by design I explain it with two interactive demos joanleon.dev/en/crux-28-d... #WebPerf #CrUX
In my audits, reading the Performance profile is where people get stuck: the flame chart and waterfall look like noise at first I built an interactive guide to make them click, every figure is interactive, plus LCP, CLS, INP and the Insights panel perf.reviews/profile-guide #WebPerf #DevTools
Reading a Performance Profile: Field Guide
An interactive field guide to the Chrome DevTools Performance panel: waterfall, flame chart, LCP, CLS, INP, and the Insights panel.
perf.reviews
An occupational habit: I open DevTools on every site I visit, hunting for things to improve. On skills.addy.ie I caught a 301 on every internal click, a wasted round trip. It was a trailing-slash mismatch. Wrote up the finding and the fix: joanleon.dev/en/trailing-... #WebPerf #Astro #Netlify
“Most teams fall into a reactive performance decay cycle: ship, wait for complaints, panic, patch, repeat. The Fast by Default model provides a repeatable alternative that keeps systems healthy.” Performance Engineering in Practice, by @denodell.bsky.social #WebPerf #Performance
Llevo tiempo viendo demos de multiagentes y casi todas sufren de lo mismo: un agente escribe y otro revisa... para acabar haciendo una todo-app No son malas demos, pero no hay un motivo técnico real para dividirlas en agentes. Así no aprendemos cuándo sí tiene sentido Hilo con una alternativa 👇
Safari and Firefox change how big sites render based on the domain. TikTok, Netflix, Instagram… even SeatGuru. Chrome doesn’t. Why is that? denodell.com/blog/browser... by @denodell.bsky.social #Browser
Browsers Treat Big Sites Differently
Safari and Firefox change how big sites render based on the domain. TikTok, Netflix, Instagram… even SeatGuru. Chrome doesn’t. Why is that?
denodell.com
¿Buscas optimizar el rendimiento de tus aplicaciones con agentes de IA? En este workshop aprenderás a conectar el Chrome DevTools MCP a un agente para automatizar la detección de problemas. Veremos cómo generar reportes técnicos, de cliente y de management 👇 ¡Inscríbete aquí! #WebPerf #AI #MCP
Joan León@nucliweb.net · 5mo agoEl agente de IA no sabe debuggear rendimiento si tú no le das el contexto para hacerlo. No es que sea malo. Por defecto trabaja a ciegas: sin acceso al navegador, sin trazas reales. #WebPerf #Performance #AIAgents #ChromeDevTools #Frontend
Running performance audits by hand, scripts in DevTools, results scattered, is fine until you're doing it on every project I built PerfReviews Insights to skip that. URL in, structured report out. Fonts, images, CSS, LCP, TTFB. Pro accounts keep full history perf.reviews/insights #WebPerf
PerfReviews Insights — Web Performance Analyzer
Analyze any website and get an instant performance report: images, videos, third-party scripts, and page weight.
perf.reviews
El agente de IA no sabe debuggear rendimiento si tú no le das el contexto para hacerlo. No es que sea malo. Por defecto trabaja a ciegas: sin acceso al navegador, sin trazas reales. #WebPerf #Performance #AIAgents #ChromeDevTools #Frontend
WebPerf Snippets meets AI agents: Orchestration without hallucination slides.com/joanleon/web... #WebPerf #WebPerfSnippets #PerfTools
WebPerf Snippets meets AI agents: Orchestration without hallucination
WebPerf Snippets evolved from manual DevTools scripts into an open-source project. Now, it explores a nervous question: Can AI help with web performance? The answer is yes, provided the LLM doesn't m...
slides.com
Join us tomorrow for two great talks from Alfonso Moure and @nucliweb.net ! #webperf All details at... performanceobserver.dev
PerformanceObserver: A virtual web perf meetup
Join the PerformanceObserver virtual performance meetup to get all the latest web performance news and hang out with the community.
performanceobserver.dev
I've finished the #WebGPU in the browser series. 4 articles: fundamentals, image processing, real-time ML, and video editing Never written a shader before starting. In the last article we process video at 2ms per frame with WebGPU + WebCodecs joanleon.dev/en/webgpu-br... #WebPerf #JavaScript
WebGPU: the browser's new performance engine | Joan León
WebGPU exposes the GPU directly to the browser, opening new possibilities for intensive processing: images, video, ML, and more.
joanleon.dev
Toggle CSS classes in bulk, then reset the scroll: two lines that force the browser to block the main thread synchronously. I documented how to detect it in the Performance Panel and why one requestAnimationFrame is not enough to fix it. #WebPerf #CSS #DevTools #JavaScript
The DebugBear custom dashboards feature is live now! We provide a flexible interface where you can display the web performance data that matters to your team. youtu.be/96fFkixiC8Y
How to create custom web performance dashboards for your team
YouTube video by DebugBear
youtu.be
Thanks to Codemotion for giving me the opportunity to continue sharing my knowledge on #WebPerf (aka improving the UX), and to do so in a workshop format where we use Chrome DevTools MCP to identify performance issues on a website with AI agents. #WebPerf #PerfTools #DevTools #CodemotionMadrid
A Gaussian blur in Canvas 2D takes 280ms. The same filter with WebGPU: 8ms. I wrote the second article in the WebGPU series: image processing in the browser with WGSL shaders and real benchmarks. joanleon.dev/en/webgpu-im... #WebPerf #WebGPU #JavaScript #Performance
Image processing in the browser with WebGPU | Joan León
How to use WebGPU to apply filters and transformations to images in the browser. Real benchmarks comparing with Canvas 2D.
joanleon.dev
Joan León@nucliweb.net · 6mo agoFor years, the performance ceiling in the browser has been clear: the CPU. WebGPU changes that. 🧵 It’s not a library or a framework. It’s a browser API that gives direct access to the device’s GPU, enabling a level of parallelism the CPU cannot match. #WebPerf #WebGPU
I wrote about the experimental chrome-devtools-mcp CLI: a persistent daemon and direct subcommands to control Chrome from the terminal without writing code. Network, performance, Lighthouse, memory, interactions. All from the shell. joanleon.dev/en/chrome-de... #WebPerf #DevTools #JavaScript
Automate the browser from the terminal with Chrome DevTools MCP CLI | Joan León
The chrome-devtools-mcp package includes an experimental CLI that lets you control Chrome directly from the terminal: navigate, interact, audit performance, analyze network and memory, all without lea...
joanleon.dev
Just published the first article of a series. Covers what WebGPU is, CPU vs GPU for performance, and when to use it (vs WebAssembly). Next: image processing, ML inference, and real-time video, with real code and benchmarks. 👉 joanleon.dev/en/webgpu-br... #WebPerformance #WebGPU #JavaScript
WebGPU: the browser's new performance engine | Joan León
WebGPU exposes the GPU directly to the browser, opening new possibilities for intensive processing: images, video, ML, and more.
joanleon.dev
Sometimes opportunities come along that push you out of your comfort zone. An international product, millions of visits, and impact at scale. Definitely an exciting challenge. #WebPerf #CoreWebVitals #PerfChallenge
