Why it matters
HTML, CSS and JS compress 70-90% with brotli or gzip; uncompressed transfer multiplies download time.
How to fix it
Enable brotli (or zstd) at the CDN/reverse proxy for text/html, text/css, application/javascript, application/json and image/svg+xml.
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.
image-oversizedImage is much larger than its rendered sizeimage-legacy-formatImage served in a legacy formatimages-missing-dimensionsImages without width/height or aspect-ratiorender-blockingRender-blocking resources delay first paintcache-headersHashed static assets are not cached long-termthird-party-heavyHeavy third-party footprintdom-sizeDOM is very largebfcache-blockedPage is not eligible for the back/forward cache