I get up to 20KB/s download with encryption turned on, and only 8KB/s with it turned off.
Where are your seeders located? SA? on 41.208.xxx.xxx IP ranges?
I've done various experiments that prove iBurst interfere with torrents. One of my experiments involved seeding from the iBurst node, to MWEB ADSL, which had the same effect as mentioned, 20kb/s with "protocol obfuscation" turned on.
iBurst interfere with the ability to connect to peers internationally, injecting bogus TCP responses and more. and I have done side-by-side protocol analysis using the identically same torrent. The moment someone locally (in SA) begins seeding the file during the course of normal bittorrent operation, it downloads on iBurst. Since my interests do not coincide with the South African masses, my torrents remained dead until the day I connected my ADSL line and they came alive and I was able to complete the downloads. So therefore, totally useless to me. That is why the VPN was so critical, it enabled me to download stuff that most people in SA are just not interested in.
At one stage I asked friends to download the torrent so I could connect to their computer to leech from them. Pretty pointless they could have downloaded the whole thing and sent me a DVD in the mail.
I can re-do the experiment at any time under even more controlled conditions.
iBurst don't just nail BitTorrent either, they also agressively nail eMule, which is a total disaster, as you will see when you try to run eMule. You will notice how search results are sparse, and servers refuse to accept connections from your client.
They also throttle video streaming from YouTube and Facebook is also choked.