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The gzipped size is what actually gets transferred over the network when a user loads your site. Servers compress JS files with gzip before sending. The raw (minified) size is what gets parsed and executed by the JavaScript engine. Both matter: gzip determines download time, raw size affects parse/execution time on slower devices.

dayjs (3 kB gzipped) vs moment (69 kB gzipped) — a 23x difference. Choose dayjs for basic date formatting without complex timezone handling. dayjs uses the same API as Moment.js so migration is minimal. Only use moment if you need heavy locale/timezone features that dayjs plugins cannot provide.

Tree-shaking removes unused exports — it helps with ESM libraries like lodash-es but not CJS libraries that export a single object. Moment.js cannot be tree-shaken because of how it registers locales. Check if the library has a module field in package.json (ESM build) — that is a prerequisite for effective tree-shaking.

Each dependency can add transitive dependencies (dependencies of dependencies). Libraries with many dependencies increase the chance of version conflicts, security vulnerabilities, and bundle bloat. Prefer libraries with zero or few dependencies for critical performance paths.

Google Core Web Vitals guidance: total JS under 300 kB gzipped for good LCP. Per library: under 5 kB is tiny, 5-25 kB small, 25-100 kB medium, over 100 kB should trigger a review. Use dynamic import() to lazy-load large libraries not needed on the initial render.