Definitely not. Page load (thats the time the browser spinner stops spinning), first render, etc. are slightly better metrics. But even page load time and firs

Definitely not. Page load (thats the time the browser spinner stops spinning), first render, etc. are slightly better metrics. But even page load time and first render times aren't without their own problems. Resources loaded asynchronously also count towards the page load time - however, they are not necessarily detrimental to the user experience. Similarly, some pages flash a white screen before contents actually starts to appear. In these cases, the first render also becomes meaningless since the user doesn't care about the "paint" of a white screen. Recently, other metrics such as the speed index and [RAIL](https://developers.google.com/web/tools/chrome-devtools/profile/evaluate-performance/rail?hl=en https://developers.google.com/web/tools/chrome-devtools/prof...) have come about. (Just the initial page load time does not matter for a good UX) Mostly there is no one universal metric for all websites. Think about what your user wants to do on your site and optimise for that metric. For example, Twitter optimises for the time for the first tweet to appear on the timeline :)