A slow website, measured before anything changes.

The most common causes are images larger than needed, scripts from extensions and outside services, a missing cache, a server that cannot cope and a cluttered database. Usually two or three are at work at once. Which one weighs most differs per site, which is why it starts with a measurement instead of an assumption.

Only if the server is the cause. You recognise that by a long wait before anything appears on the screen at all. If the page arrives quickly but takes a long time to become usable, the problem is in the images and scripts, and another host changes little. We measure the two parts separately.

Google uses three measurements, the Core Web Vitals: how quickly the largest element is on screen, how quickly the page reacts to a click and whether the layout jumps while loading. You measure them in a test environment and with real visitors. The second counts for more, because a test on a fast laptop says little about an old phone.

Yes, speed for real visitors is one of the signals Google weighs. It is not the heaviest signal: a slow page with the best answer beats a fast one with a weak answer. The direct effect on visitors is bigger. Someone who has to wait long on a phone has gone before the page appears.