Earlier quoted context omitted.
Make a copy of your profile (to preserve your history) then in Firefox's Show All History window, search for a site that has thousands of visits, select them all, then press delete. Firefox will start burning a few CPU cores and lock up for a long time, every time.
That might very well be performing poorly. But I interpreted the OP to talk about usability of viewing all the previous history. My goal is to never delete my firefox "sites visited" history so I can refer to it back later. So naturally I've never experienced the problem you're describing.
Search is quick, in Ctrl+H and the URL bar suggestions are instantaneous. No complaints there.