Torrent is a P2P (Peer to Peer) network , means there is no central server to send data for clients , instead the clients connect to each other directly and communicate the data , in such network the speed depends on many factors despite the fact how much is your internet connection download speed.
if a torrent has many leechers (downloaders) but few seeders (uploaders) , then the speed of download would be low. I guess the reason is obvious
2.Network routes between peers
different geographic locations are connected to each other with different connection types , some with fiber optics , some with satellites , some with wireless networks and etc.
so it really matters where the seeders and leechers are located. someone in US may get 10x speed rather than someone in EU for a same torrent just because of network routes.
The torrent softwares ( Torrent Clients ) have different performances , I would recommend uTorrent for best performance.