usually depending on how well seeded they are, it will try to give the highest seeds about 3/4 of the speed ... then again alot of seeds doesn't always mean the highest speeds. private tracker torrents are usually the fastest no matter what though. i suggest you try http://www.thepeerhub.com/ for some fast speeds. it can never go above your max speed tho, so each one will get some out of the max speed.
1. Your internet speed. With 5mbps internet connection, your maximum download speed will be as such. What this means is that if you are downloading several things that take up more than 5m of bandwidth per second, your internet speed will be throttled down.
2. The amount of seeders of the torrent. Without a good amount of seeders, the connection speed will be slow regardless.
yeah. you only have a certain amount of bandwidth to work with, so the torrent program would have to try to evenly distribute said bandwidth evenly to each download in progress.
I.E. if you have 2 downloads running and lets say your max download speed is 100kbps, it would split the bandwidth and give each download 50kbps.