Why it matters
JavaScript is the most expensive byte: it must be downloaded, parsed, compiled and executed on the main thread. Marketing pages over ~170 KB compressed are slow on mid-range phones.
How to fix it
Render static UI on the server, lazy-load below-the-fold widgets, replace heavy libraries (moment, lodash, chart libs) and load third-party scripts on interaction or idle.
References
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.
cls-highCumulative Layout Shift is highttfb-slowServer response (TTFB) is slowfcp-slowFirst Contentful Paint is slowtbt-highMain thread is blocked by long tasks during loadcss-budgetCSS transfer exceeds the budgetfont-budgetWeb font transfer exceeds the budgettoo-many-fontsToo many font files are downloadedimage-budgetImage transfer exceeds the budget