Joan León
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nucliweb.net
Joan León
@nucliweb.net
1.9K followers 510 following 660 posts
⚡️ Web Performance Artisan at @perf.reviews | Google Dev Expert in Web Tech | Cloudinary Ambassador | ❤️ Image Optimization | Girona
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nucliweb.net
Joan León @nucliweb.net · 19d
WebPerf Snippets started as a collection of snippets to paste into the DevTools console. With version 2.0, they are also built for AI agents to run

webperf-snippets.net

🧵 #WebPerf #AI
WebPerf Snippets
Tested JavaScript snippets that measure and debug web performance. Your AI agent runs them for you, or you run them yourself in DevTools: the same code every time.
webperf-snippets.net
Reposted by Joan León
tunetheweb.com
True!

Lighthouse 13.6.0 (now on PSI and will be in Chrome 159—currently in Canary) improves the simulated FCP and LCP resulting in more accurate—usually better—scores.

Note this is not SPA-specific (and unrelated to other SPA things we've been working on), but more likely to impact JS-heavy sites.
maxwellcohen.bsky.social Max @maxwellcohen.bsky.social · 2d
@developer.chrome.com recently updated Lighthouse, resulting in SPA apps getting much better results. Now @react.dev has a perfect performance score! Like many SPA sites, react.dev was reporting LCP (~7s) and TBT (~2s), but now it's 1.1s (LCP) and 10ms (TBT)!
pagespeed.web.dev/analysis/htt...
react.dev perfomance section on moblie
November 2, 2026 at 4:30 PM
Reposted by Joan León
veneman.dev
github.com/projectwalla... all ready for the Stylelint v18 release 👀
GitHub - projectwallace/stylelint-plugin: A stylelint plugin pack that checks the complexity of your CSS.
A stylelint plugin pack that checks the complexity of your CSS. - projectwallace/stylelint-plugin
github.com
November 2, 2026 at 8:17 PM
Reposted by Joan León
a11yconf.com
💙 ¡Últimos ponentes de la a11yConf 2026!

🌳 El árbol de accesibilidad frágil: por qué el HTML semántico es clave para el rendimiento web
🎙️ @nucliweb.net

¿Sabías que el HTML semántico no solo ayuda a crear experiencias más accesibles, sino que también puede hacer que nuestras webs sean más rápidas?
a11yconf 28 noviembre, Zaragoza El árbol de accesibilidad frágil: por qué el HTML semántico es clave para el rendimiento web Joan León
November 2, 2026 at 5:47 PM
Reposted by Joan León
perfnow.nl
With 2 years of rollouts, we've learned a lot about compression dictionaries and the Web. Patrick will cover what works, where the footguns are, how to maximize their effectiveness, and other experiments around compressing content on the web.

@patmeenan.com at perfnow.nl
Latest on Web Compression by Pat Meenan, Google at performance.now(), 19-20 November in Amsterdam
October 21, 2026 at 10:27 AM
Reposted by Joan León
perfnow.nl
Sometimes custom solutions that we build highlight deeper challenges with performance and usability, and reveal an opportunity to grow the web’s native capabilities. How doe you move from patterns that work on one website to features that are built into our browsers?

@scottjehl.com at perfnow.nl
Standards from the Field by Scott Jehl, Squarespace at performance.now(), 19-20 November in Amsterdam
October 22, 2026 at 11:25 AM
Reposted by Joan León
machal.bsky.social
Getting web performance prioritized is the hard part.
I will talk about this topic on @perfnow.nl.

How do you make the case for web performance?

→ Share your experience in this 3–5-minute survey: pagespeed.one/en/perfnow

🙏 #WebPerf #WebPerformance #PageSpeed
October 15, 2026 at 6:10 PM
Reposted by Joan León
twnsnd.com
We now have LCP and INP sub-parts in @cloudflare-dev.bsky.social Real User Monitoring 🎉

This gives you deeper insight into where your websites and web applications are causing frustration for your users, so you can target changes more effectively.

developers.cloudflare.com/changelog/po... #webperf
Deeper Core Web Vitals Diagnostics: LCP & INP Sub-Part Metrics in Real User Monitoring
Real User Monitoring now breaks LCP and INP into their sub-parts and splits connection time into TCP and TLS. This helps you find the cause of slow pages in the dashboard and through the GraphQL Analy...
developers.cloudflare.com
October 13, 2026 at 11:25 AM
Reposted by Joan León
perfnow.nl
In this deep-dive session, Tim will use real-world data and performance metrics to show how scalemates.com became the fastest and largest scale modelling website in the world. You’ll leave with practical, actionable insights to push performance even further.

@timvereecke.bsky.social at perfnow.nl
Beyond the P75 by Tim Vereecke, Akamai at performance.now(), 19-20 November in Amsterdam
October 20, 2026 at 10:52 AM
Reposted by Joan León
gdggirona.bsky.social
🚀 From population models to personalized AI!

How can we build digital twins tailored to individuals?

Join #GDGGirona on Oct 22 for a live demo exploring generative deep learning, transfer learning & personalized healthcare

👉 gdg.community.dev/events/detai...

#AI #MachineLearning #DigitalTwins
From Population Models to Personalized Digital Twins | Google Developer Groups
In-person Event - Join us for a session on enhancing machine learning accuracy for individuals using generative deep learning and transfer learning. Explore personalized healthcare applications like s...
gdg.community.dev
October 17, 2026 at 8:16 AM
nucliweb.net
WebPerf Snippets started as a collection of snippets to paste into the DevTools console. With version 2.0, they are also built for AI agents to run

webperf-snippets.net

🧵 #WebPerf #AI
WebPerf Snippets
Tested JavaScript snippets that measure and debug web performance. Your AI agent runs them for you, or you run them yourself in DevTools: the same code every time.
webperf-snippets.net
October 16, 2026 at 1:22 PM
nucliweb.net
Es todo un lujo poder formar parte de @a11yconf.com como ponente, donde hablaré de "El árbol de accesibilidad frágil: por qué el HTML semántico es clave para el rendimiento web"

🎟️ Aprovecha ahora que quedan entradas!!!

2026.a11yconf.com/entradas/

#a11y #WebPerf #a11yConf
Entradas - a11yConf 2026
La conferencia de accesibilidad web más importante de España. Únete a profesionales, diseñadores y desarrolladores comprometidos con la inclusión digital.
2026.a11yconf.com
October 14, 2026 at 6:19 PM
nucliweb.net
Thank you so much @veneman.dev 🥰

Two awesome new stickers on my laptop from a great project: @projectwallace.com

#CSS #CSSTools #WebPerfTools
A laptop covered in colorful tech stickers, including two new stickers: a bright CSS green “CSS” design over a lightning bolt and a black “CHAOTIC STYLE SHEETS” sticker with the same green lettering drawn over the white text. Other stickers feature web development tools, browsers, performance testing, and developer communities.
October 9, 2026 at 2:33 PM
Reposted by Joan León
foolip.org
JPEG XL shipped in Chrome 155 today, try it out!

Huge kudos to Luca and team for getting this done. Image codecs are cool in so many levels, and I'm super impressed by the people who make that magic happen.

developer.chrome.com/blog/jpeg-xl...
Shipping JPEG XL in Chrome  |  Blog  |  Chrome for Developers
JPEG XL offers better compression than JPEG, built-in HDR support, lossless JPEG transcoding, and more.
developer.chrome.com
October 6, 2026 at 7:09 PM
nucliweb.net
During a perf audit for a client, I read this in a PR: "The module should only be loaded once, not every time a file is selected." When I read it, something didn't add up, so I started digging. And the result surprised me.

I wrote a post joanleon.dev/en/dynamic-i...

#JS #WebPerf
Dynamic import(): the browser already caches our modules | Joan León
Calling import() every time a feature is used doesn't download or evaluate the module again: the browser reuses it from its module map. What it guarantees, when the same module stops being the same, a...
joanleon.dev
October 5, 2026 at 9:44 PM
Reposted by Joan León
benschwarz.bsky.social
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
September 29, 2026 at 5:48 AM
nucliweb.net
One thing I’m experimenting with for a performance audit:

Using RUM data + @debugbear.com MCP to build synthetic tests based on how the product is actually used

RUM → real user behaviour → MCP → representative synthetic tests

www.debugbear.com/docs/mcp

#WebPerf #RUM #MCP
Access Web Performance Data With The DebugBear MCP Server | DebugBear
Connect AI assistants like Claude and ChatGPT to DebugBear using the Model Context Protocol (MCP) to run tests, query metrics, and manage projects.
www.debugbear.com
September 29, 2026 at 6:25 PM
nucliweb.net
Thank you to @en.sitnik.es for the @evilmartians.com sticker ☺️
September 29, 2026 at 7:17 AM
nucliweb.net
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
A dark-themed line chart titled “SSIMULACRA2,” showing BD-Rate (%) versus encoding speed in milliseconds for several image codecs. Colored lines compare codecs including AOM, JPEG XL, aperture-alpha, Iris-WebP, libjxl, SVT-AV1, and libwebp, with labeled data points. Most curves show higher BD-Rate at slower speeds, while some reach negative BD-Rate at faster speeds.
September 20, 2026 at 2:04 PM
nucliweb.net
I added WebMCP to the GeeksCAT Conf 2026 website, so an agent can ask for the schedule, the speakers or the event details as tools instead of reading the HTML.

It's a static Astro site, so I go through it layer by layer.

joanleon.dev/en/webmcp-st...

#WebMCP
WebMCP on a static site: the GeeksCAT Conf 2026 website, ready for agents | Joan León
The GeeksCAT Conf 2026 website is a static Astro site that exposes its schedule, speakers and event details to AI agents through WebMCP. Three layers: a manifest in /.well-known, JSON endpoints genera...
joanleon.dev
September 16, 2026 at 6:32 PM
Reposted by Joan León
developer.chrome.com
We created Modern Web Guidance to bring web platform expertise right into your agent workflows. Have you tried pairing these skills with Chrome DevTools for agents to audit performance and automatically fix code?

Check it out and join 10K+ web developers who are coding smarter → goo.gle/4yEVz1h
September 16, 2026 at 6:30 PM
nucliweb.net
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
DebugBear RUM dashboard showing Core Web Vitals at the P75 percentile: Largest Contentful Paint 1.58s, Interaction to Next Paint 176ms, and Cumulative Layout Shift 0.01, all flagged green on the score bar. DebugBear RUM dashboard showing Core Web Vitals at the P90 percentile: Largest Contentful Paint 2.68s, Interaction to Next Paint 352ms, and Cumulative Layout Shift 0.17, all flagged orange or red on the score bar.
September 7, 2026 at 10:17 AM
Reposted by Joan León
webperfreport.bsky.social
⚡Featured posts this week:
✨ Submit your proposals for Interop 2027: @rachelandrew.co.uk
✨ Reading a performance profile: @nucliweb.net
✨ HTML over WebSockets: @androsfenollosa.bsky.social
✨ Intro to Web UX and Speed Optimization: @sergeyche.dev
🙏 Special thanks to the authors!
#webperf
WebPerformance Report performance email preview highlighting optimization scores, Perf Links, and Core Web Vitals metrics on a colorful gradient background.
September 6, 2026 at 12:58 PM
Reposted by Joan León
tomayac.com
🤩 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
August 28, 2026 at 3:12 PM
Reposted by Joan León
webperfreport.bsky.social
⚡Featured posts this week:
✨ AI and organic search are doing different jobs: @kylerisley.bsky.social
✨ The CrUX 28-day rolling window: @nucliweb.net
✨ AI Web Readiness as a Curb-Cut: @ethangardner.com
✨ HTTP Integrity Digests: @simmervig.org
🙏 Special thanks to the authors!
#webperf
WebPerformance Report performance email preview highlighting optimization scores, Perf Links, and Core Web Vitals metrics on a colorful gradient background.
August 16, 2026 at 7:00 PM
nucliweb.net
I love the optimization: web, rendering, images, video, and now tokens.

@lydiahallie.com published a great guide to Maximizing the value of your Claude Code sessions

claude.com/blog/maximiz...

#AIOptimization #AI #Claude
A dark-themed presentation slide titled "TL;DR" displaying tips for optimizing Claude API usage and token efficiency. The slide is divided into two columns: Left column discusses: - Using `/clear` command between tasks to prevent context carryover and reduce token usage - Setting model and effort level before starting a session to avoid breaking the prompt cache mid-conversation - Mentioning files with @ notation instead of naming them, to attach files directly and save API calls Right column covers: - Adding quiet flags to commands or running them in a subagenr to manage noisy output while keeping results in the conversation - Running `/context` once per session to review loaded content (CLAUDE.md and MCP tool definitions) and remove unnecessary items - Running `/compact` before breaks, since prompt cache expires after an hour and summarizing conversations is cheaper while the cache is active
August 16, 2026 at 9:43 AM