The first thing to understand is that there is no single metric or measurement for ‘speed’. There’s no simple number which you can use to measure how quickly your pages load.
Think about what happens when you load a website. There are lots of different stages and many different parts which can be measured. If the network connection is slow, but the images load quickly, how ‘fast’ is the site? What about the other way around?
Even if you try to simplify all of this to something like “the time it takes until it’s completely loaded“, it’s still tricky to give that a useful number.
For example, a page which takes longer to ‘finish loading’ may provide a functional ‘lightweight’ version while the full page is still downloading in the background. Is that ‘faster’ or ‘slower’ than a website which loads faster, but which I can’t use until it’s finished loading?
The answer is, “it depends”, and there are many different ways in which we can think about or measure ‘site speed’.
>> Read : How to check site speed
Source : Yoast