The speed can depend on how much you seed or leach if the torrent site is private and has rules that if for example you have a rating less than 0.2 or smth like that you will be limited to less downloads at a time or limit the speed to #kb/s. Public sites that do not require u to register to download will not do that.
The speed firstly depend on your ISP speed.
After that, it depends on how many Seeds and Leeches there are.
If there are no seeds you wont get a download, if there are no seeds but some leeches exist you will not get all the download. If there are more leeches than seeds the speed would likely be slower than vice versa.
No, it depends on the peers connected to the torrent and their upload capabilities as well as your own bandwidth capabilities. Seeding more or less has nothing to do with the speed.