Considering this doesn't happen for everyone, it can only really be your internet connection or your computer. Internet speeds is the obvious one, however how you have your computer configured or the hardware itself could be slowing it down. For example, if your CPU is maxing out, then it'll slow down your web browser, not specifically the forum.
I use old hardware, and I notice the slow downs from time to time due to CPU load. Although, that's also because I'm running Qubes OS which is notoriously slow due to the fact its running multiple Virtual Machines, although this probably isn't the case for you, have a look to see if there's anything else slowing it down or just check your CPU load during browsing.
Tor isn't that slow. I use Tor, but most thread fully loaded in less than 15 seconds.
I think its old reputation has stained its modern day reputation. Tor is pretty decent nowadays, and his a notable improvement to what it was five plus years ago. The only issues you get is with Captcha's which isn't exactly Tor's fault.
Obviously, it doesn't perform as good as other browsers when loading complex pages, but Bitcointalk is anything, but complex.
