Hi, I am having similar problems, been using the transmission addon for a while now, last week I decided to upgrade my main hard drive so I replaced it and started from scratch, updated firmware to 1.04b12 while I was at it. After I finish setting everything up and started using transmission I noticed I was getting very slow download speeds, they don't go up from 50 kb/s, so I used the same torrent on my windows PC and the speed was fine, maxing my dsl speed. So i know it must be related to transmission, to confirm it I installed rutorrent and it is working fine, downloading at max speed. I don't know what to do. Maybe it has something to do with the new firmware?.
P.S. Using DNS-320L. I already check if encryption was turned off and ports are open.
Thanks.