JavaScript execution cost
The on-device CPU time needed to parse, compile, and run JavaScript, which can hurt even after a small compressed bundle downloads.
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What it is
JavaScript execution cost is the device time spent parsing, compiling, and running shipped code. Download size predicts some of it, but code shape and device speed matter too. A compressed bundle can cross the network quickly, expand into much more source, and then occupy the main thread while the interface waits. This is the point Addy Osmani's 'The Cost of JavaScript' hammered on: byte for byte, script is the most expensive thing a page can download, because an image only has to be decoded while a script has to be parsed, compiled, and run.
Reach for it when a page paints but taps lag, startup stalls after the network goes quiet, or desktop looks fine while a low-end phone struggles. Chrome DevTools Performance exposes long tasks and its Bottom-Up view shows which functions consume CPU. Profile with CPU throttling or representative hardware, not only a development laptop.
Gotcha: moving code is not removing cost. A delayed script can still freeze the first interaction, and splitting one long task into queued work can monopolize the thread. Delete work first, yield between remaining chunks, and move pure computation to a Web Worker, which cannot directly read or update the DOM.
Ask AI for it
Reduce this page's JavaScript execution cost. Record startup and the slowest interaction in Chrome DevTools Performance with 4x CPU throttling, then use Bottom-Up to rank script evaluation and functions by self time. Delete unnecessary work, lazy-load optional features with import(), break tasks over 50ms with scheduler.yield(), and move pure CPU-heavy computation into a Web Worker. Keep DOM work on the main thread, repeat the identical trace, and report before and after total script time, longest task, Total Blocking Time, and INP.