I've noticed that when I download torrents on Mac OS X (using BitComet in Windows XP on Parallels -- I like it better than using Azureus in Mac OS X) and then try to browse the web, Firefox starts to time out a lot. Browsing the web becomes impossible. I installed Throttled Pro and my machine instantly became usable again. Throttled Pro features ACK packet prioritization, which is what really made the difference in my case (I set upload caps manually in my torrent clients and wasn't saturating my upstream connection, but for some reason the ACK delay was still causing network slowdown).
It's an amazing difference and is worth trying, especially if you're having network slowness caused by full-speed uploads.