If ported correctly and your bandwidth isn't being used for anything else then there really isn't nothing to be done. Your speed of course is dependent on your actual internet connection speed. 512Kb/sec isn't bad, if it seems slow it must be a huge file you are downloading. Also it depends on the upload speeds of the people you are connecting to. If they have a slow internet connection or set their torrent client to a predetermined speed you will download slowly. I have a 256Kb upload rate and download rate of 6Mb from my ISP and can download at about 750Kb/sec but it depends on those factors, sometimes I might only download at 5Kb/sec.