Performance budgets fail when they are too long. A spreadsheet with thirty metrics gets ignored by week three. The teams that ship fast websites hold themselves to a small set of numbers that everyone on the team can recite from memory.
These are the five we use.
1. Largest Contentful Paint (LCP) under 2.0s on a mid-tier device
LCP measures when the largest meaningful element renders. Below 2.5s is Google's "good" threshold; we hold ourselves to 2.0s because the user-perceived ceiling is closer to that. The two biggest wins are right-sized hero images and a server that responds quickly to the document request.
2. Interaction to Next Paint (INP) under 200ms
INP replaced FID as the responsiveness metric in 2024. The single biggest cause of poor INP scores is long-running JavaScript on the main thread — usually a third-party tag or a poorly-shipped React component. Audit your scripts and your hydration boundaries.
3. Cumulative Layout Shift (CLS) under 0.05
CLS is the cheapest metric to fix and the most jarring to users. Reserve space for every image and embed. Render the layout before the data when you can. Never load fonts without size-adjust and font-display: swap.
4. Total JavaScript transferred under 170KB compressed
This is the budget that matters most for long-tail devices and slow networks. Most teams blow it on a single large dependency, often a date library, an icon set imported in full, or a state management library that was never necessary. Bundle analyzers are free; run one on every release.
5. Time to First Byte (TTFB) under 200ms from the user's region
TTFB is your server's responsibility. If you are above 200ms, the answer is almost always edge caching for unauthenticated routes, or moving compute closer to users for authenticated ones. There is no JavaScript fix.
Hold the line in CI
Every one of these numbers can be measured in CI. Lighthouse-CI for synthetic checks, real-user monitoring for the production picture, and a hard fail on regressions in the budget. If the numbers can regress silently, they will.