sometimes it just depends on the seed/leech ratio. if theres 100 seeds, and theres 10000 leech (10000 people downloading) then its gonna be slower than one with say 100 seeds and 100 leech.
what speeds did you get on speedtest.net ? look at what your upload speed is on there then divide it by 10 and put it as your max upload speed.
also is the port your using open? look in the bottom panel and there'll be either a green tick, a yellow exclamation triangle, or a red circle with exclamation (!). if its the red one, then people arent able to connect to you which will limit your upload speed, which will in turn limit your download speed.
Maybe your ISP is throttling (speed limiting) your bit-torrent traffic... its possible.
I got around a 7mbit connection, my torrent speeds, the max i saw it on was like 350kb/s...