How to check whether a site is fast
Open it on a phone over a mobile connection, not on a computer on office wifi. The content should appear in under 2.5 seconds, and the layout must not jump while loading.
The simplest test is manual and enough in most cases. If you spend a moment looking at a white or empty page, it is too slow. If the text appears and then jumps because a graphic has loaded, that is a separate fault, and an even more irritating one.
With tools it is measured in PageSpeed Insights, but the result has to be read with judgement. What counts is the mobile tab and the data from real traffic, not the score from a simulation.
The most common causes of a slow site are mundane: photos uploaded at camera resolution, a dozen or so plugins, third party scripts and no caching. The first of those usually accounts for over half the problem.
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