Why it matters
Synchronous scripts in <head> and large stylesheets stop the browser from painting until they are fetched and executed.
How to fix it
Add defer or type="module" to scripts, load third-party tags async/lazily, and keep blocking CSS small.
Example
<script src="/app.js" defer></script>More performance rules
void perf reports 25 rules in this category. Core Web Vitals and bytes: LCP, CLS, TBT, TTFB, JavaScript, CSS, fonts, images, caching and compression, measured as the median of several runs on a throttled mobile and a desktop profile.
lcp-background-imageLCP element is a CSS background imageimage-oversizedImage is much larger than its rendered sizeimage-legacy-formatImage served in a legacy formatimages-missing-dimensionsImages without width/height or aspect-rationo-text-compressionText resources are served uncompressedcache-headersHashed static assets are not cached long-termthird-party-heavyHeavy third-party footprintdom-sizeDOM is very large