Website speed is dependent on a lot of factors - server (server load, server connection speed), client ( client connection speed, client computer spec and speed and whether some of the cached locally), web design (languages used, graphics etc).
Best practical way would be to compare the ping and response speed using several different sites against the tested site. Of course you would want to compare as much as possible against sites which host information in the same field as you are interested in.
Page speed is regularly mistaken for "site speed," which is really the page speed for an example of online visits on a website. Page speed can be portrayed in either "page stack time" (the time it takes to completely show the substance on a particular page) or "time to first byte".