I did some testing on my MIT internet with KTorrent 2.2.1 Ubuntu. I found an extremely huge speed discrepancy between DHT on and DHT off. With DHT on, this torrent with 4000 seeds and 500 peers was going at around 300/2000 KB/s. After turning off DHT, the speeds immediately jumped to around 3MB/s down and 5MB/s up. Turn DHT back on, and the same slowdown happens.

Usually I'd blame this on a poorly designed NAT router, but this is not the case. I am directly hooked up to the internet via an 18.0.0.0/8 public IP, well-capable of 12MB/s up and down simultaneously, with no tasks running.

I suspect some sort of threading bottleneck going with the network subsystem in KTorrent? I was unable to reproduce the problem in Azureus or Deluge (Deluge implements uTorrent compatible PEX and DHT)

Interesting, I'm guessing that it is probably the DHT networking code (KDE based), though obviously I need to do some experiments to verify this.

I will do some tests on a LAN next week to see if I can reproduce it.