The bittorrent protocol works like this:
While downloading bits of the requested file to your computer, you are actually uploading the bits of the file that you already have to other people... all at the same time. The amount and speed with which you can download files depends upon how much you're uploading to other people. It's a "the more you give, the more you get" type of situation.
This means that, depending upon firewalls/routers and network settings, you may not be uploading to other bittorrent clients... and therefore you won't get much back from the other bittorrent clients.
Another factor that affects download speed are the number of peers that are "seeding" a file. The more people that are actively sharing a file, the greater the likelihood of a fast download.
ANYWAY, I've rambled on about this long enough. Hope that explains why you're getting slow downloads without making your head hurt too much.
More info: http://www.bittorrent.com/FAQ.html