Opening a page versus using it
A shopping page that takes several seconds to show products has a load-time problem. If it opens quickly but stalls when you scroll or choose a filter, it has a runtime problem.
Loading speed depends on the internet connection, how much data the site sends, and how long the device takes to display it. After loading, heavy work on the device, memory use, or waiting for more data can make the page slow to respond.
Judge by what people notice
Teams test on ordinary devices and connections. They check when content appears, when buttons respond, and whether the page shifts around as it loads. A performance score is useful when it reflects what people actually experience.