Performance Lab

Save the terrible website

LCP 7.8s. INP 840ms. CLS 0.42. 4.8MB of JavaScript. You have a budget and a mission.

Main-thread blocking experiment

Run real work on your real main thread and watch your real frame rate fall over. No simulation.

The Hidden Cost of JavaScript

Transfer size is the cheapest part of shipping JavaScript. This dossier separates the four costs a script imposes, network, parse, compile, execute, and shows why the last three dominate on the devices most people actually own.

Field

Web Performance

Experiments

14

Observations

27

“A smaller bundle is always faster.”

Smaller usually helps, but *when* bytes arrive and *what* they do on arrival matter more than the total.

“100 Lighthouse means your website is perfect.”

Lighthouse is a synthetic run on a simulated device, on your network, with no real user in sight. It is a useful ratchet, not a verdict.