I guess a lot depends on what exactly you are downloading and where you are downloading it from. If it's a huge compressed file for instance, VirusScan will spend a lot of energy checking it out.
You could make sure that the bit torrent client has Full permission in Firewall > Internet Connections for Programs and you may need to play around with Ports and System Services also in that area.
If it continues you are best advised to contact Technical Support directly via the link under Useful Links at the top of this page. It's free by phone or online chat.