i second bittornado! and it's not the client that makes it slow, it's the leechers and slow uploaders and slow seeders.
when looking at torrent files you want to see more seeders and less leechers, that means there are less leechers per seeders to take away bandwidth.
it usually takes me about a full minute to get up to speed which can be anywhere from a slow 5kbps to a blazing 300+kbps, it all depends.