i just had a similar problem actually.
ran a speed test and it returned a consistent 15Mbitss but torrents were topping out at 10kBs.
heres what i did
-copy the files from the _active torrents directory. and the _torrent_files into another folder on the drive.
-go into web admin and disable the bit torrent client.
-click on initialize bittorrent settings,
-re enable bittorrent client.
-move the files back to the _active folder
-re-add the torrents through the web ui.
-make sure they force- recheck.
that worked for me.