Intuition is wrong

Everyone believes their site is slow because of "too much JavaScript". It is usually one enormous image, or a font blocking the render, or an N+1 query.

Three numbers worth knowing

• LCP, when the biggest visible thing finishes loading. Under 2.5s
• CLS, how much the layout jumps about. Under 0.1
• INP, how quickly the page reacts to a tap. Under 200ms

Where to look

DevTools → Lighthouse gives a report with the offending files named. The Network tab, sorted by size, usually answers the question in five seconds.

Throttle honestly

Test on Slow 4G with the cache disabled. Your own repeat visit on office wifi tells you nothing about a first-time visitor on mobile data in Eldoret.

The one rule

Measure, change ONE thing, measure again. Changing five things at once tells you the total but never which change mattered, and one of them may have made it worse.