Transmission is just slow. When I'm in Boot Camp/Win7, same computer, same router, using uTorrent I easily get 2-3 MB/s. In OS X with Transmission, configured correctly (the assigned port is listed as open in the preferences) and with identical settings for min/max speed up and down, the exact same torrents take 3-4 times longer. It's not merely due to the lower peak speed (it maxes out around 1 MB/s), the real problem is that the speed is so inconsistent. In uTorrent on PC it starts out slow and then it just climbs and climbs and *boom*, the file is home. In Transmission you can take a glance and it shows 800 kB/s, not too shabby, but five seconds later it's down to 25 kB/s and stays there for minutes.
The difference is less pronounced when you're downloading some old torrent with more seeders than leechers, then it's almost as fast as uT, but when you're with thousands of others fighting over a torrent that was just released, it's as if uTorrent on Windows somehow elbows its way to the front of the line, while Transmission on OS X gets trampled by everyone else and has to wait in the back of the line until the concert is nearly over.