Safari has become very slow, lagging and not loading pages

In the past couple of months I've found Safari incredibly slow, either not loading or hanging on pages etc, and its getting worse. Mostly its a question of the blue line at the top just stopping a quarter or halfway in; occasionally the grey screen comes up to say it couldn't connect to the server. It seems to happen most on facebook but now it's happening almost every time I use my machine. If I reload the page a few times this often works so it appears to be a constant but temporary problem. I've tried other browsers but they seem to have the same issue. My internet speed tests show excellent speeds are available and the problem is the same whether I'm connected via ethernet or WiFi (same issue happens with my desktop Mac as well as my laptop). I've deleted cookies, emptied caches, even changed the DNS server and renewed the DCHP lease. etc. You can probably tell I have a limited understanding of such things so I'm at a loss what to do. Is it something to do with me not having enough memory on my Mac?