I use FireFox on Mac, and the forum has frequently been unbearably slow. I finally decided to start troubleshooting, and believe I’ve nailed it. It appears to be related to “hardware acceleration” which FF uses by default (maybe other browsers do too, I’m not sure).
If you use Firefox and notice it’s very slow to open thread, reply to things, etc. try this:
Tools > Options > Advanced > General > Browsing: disable “Use hardware acceleration when available”
If you’re on Mac, it’s under Preferences not Tools.
The forum is like greased lightning now. Everything happens in under a second, it’s great.