As a matter of interest, which Internet Service Provider do you use and also which package? I have heard of certain ISP’s delibertly throttling the bandwidth of uncommon port ranges such that priority is given to the main ports such as POP3 & SMTP(e-mail), HTTP (web access), etc. This is likely the case if the speeds have slowed down all the sudden and you have not made any changes to potentially cause this such as install a Firewall (or modify an existing one’s settings), update your router’s firmware, etc.
First, just check that incoming connections are working fine. When you download content over BitTorrent in Azureus, a smiley will appear next to each download. These should turn to a green happy face after a short period (usually within a minute). If not, there is an issue accepting incoming connections.
From my experience (with a 1Mb down / 128k up ADSL link), the uplink limit should be set such that it does not severely cripple your browsing performance, but not too low either. For example, if I set the uplink limit to 12KB/sec (or higher), downloads go down to crawl and browsing performance feels worse than dial-up. An uplink limit of 4KB/sec generally works quite well and browsing performance also is about as good as just a HTTP based download going in the background. Finally, an uplink limit of 8KB/sec seems to give the best balance for downloading. Then again, while this works well for me with my ISP, someone with a faster uplink (such as 256K) would benefit with higher upload limit figures.