What browser are you running? I've recently (the last month) noticed Safari running REALLY slow after it's been online for a while. In Activity Monitor, I see that its use of CPU has gone from 250MB RAM up to over 1GB. Quitting Safari and relaunching helps dramatically, but after two hours or more, it repeats. (I'm on Safari 4).
You might notice Firefox behaves better for you. Do you find Mail runs normally?
The other issue is that wifi is considerably slower than Ethernet, so if you can hook a cable to the router you might notice an improvement.