Why do websites take so long to open, or freeze?

I'm constantly dealing with sites taking forever to open (including this one), or for Safari to completely freeze, nothing responding, clicking on anything does nothing, and I just have to wait it out, usually a couple minutes. I'll check my WiFi connection, and speed, but it will show it as anywhere from 60 to 110 Mbps! So it's not my connection, it's something about Safari. And it's terrible with Facebook (or vice versa), as one FB page will require from 1 to 3 GBs of RAM, and there's almost the constant banner message, telling me this page is using a lot of memory, and I might want to close it. Well, I still have a lot of memory, and a fast connection, so why the slowness and freezing?