I'm using Ubuntu 11.10 and bittorrent download speed is very slow. I tried many torrent clients, opened ports but nothing works.
I don't know what information you'll need for this problem so tell me what to do and I'll post them.
sometimes some isp's will throttle your bittorrent traffic. some isps can be tricked by forcing your bittorrent traffic to another port and encrypting it.
what i do is force encryption and use the openvpn port tcp/1194 for my communications. this seems to work for some places.
Try to set the bandwidth that your bit-torrent client is allowed to use, to a value that is about 5% less (depends on connection speed) than you have available on your Internet Connection.
You could be experiencing a networking problem known as buffer bloat.
The idea is to prevent the buffers in your router from getting full.
This is probably because the number of concurrent connections is too high for your home/office router to handle. Try setting a lower maximum number of connections (or number of "peers") in your BitTorrent application.
This is commonly the cause of a whole Internet connection being slower, but it also often causes BitTorrent downloads to be slow (sometimes even when the Internet connection, for other uses, is reasonably fast).
There are more details in the answers to this question (which is related but not a duplicate, as it's specifically about the effect of BitTorrent on other Internet activity):
Try lowering the total number of connections to somewhere around 20. (That is, not for each torrent, but for all torrents combined.)
If that helps, you can adjust it further (both up and down) to see what gives you the best performance.
If this does not help you, then please provide more information:
(Hopefully, limiting the number of connections will just work though, and the problem will be solved. Hopefully.)