what is the major difference between page speed and site speed
I’d think they’re the same.
ladans37 is right they are often the same. When people measure a site, they are often measuring how fast the assets of a page load from a given page (usually the home page). By loading the home page, you can get an idea of its weight (how many KB/MB worth of assets are downloaded) how fast the site responds (Often referred to as the time to first byte) and how quickly the assets are transferred back (things like if they are parallel loaded or is the given page held up by the loading of a script).
There are even more factors that go into how fast a page loads, but that is what you are really measuring when you saying “site speed”. They are almost always identical when people are talking generically.