http://airvpn3epnw2fnsbx5x2ppzjs6vxtdarldas7wjyqvhscj7x43fxylqd.onion/forums/topic/23458-torrent-download-speed-went-from-3~4mbs-no-airvpn-connection-to-300~1000kbs-airvpn-connected?tab=comments
That was about 80% drop in performance. I have set up remote port forwarding, disable UPnP port mapping and NAT-PMP. It didn't help. I think since I could reach to 38~ mbps on Youtube with AirVPN, the only two explanations will be either AirVPN is not friendly to p2p, or my Torrent client and setting is not friendly to AirVPN.