It simply may be the website that's slow, Also without any more information like the website in question no one can test it on their end to try and replicate the problem or experience the issue them self's.
Like Brian said, suggesting some of the websites to investigate
would be helpful.
If it's every website you interact with, such a pervasive
problem would inevitably have been noticed by our beta testers. At
this point I'd look at aspects of your computer (what type? what
speed? how much RAM? which version of OS X?), what system
extensions or login items you're using, and what add-ons are
installed in TenFourFox.