The tor network essentially relies on donated exit node bandwidth, and there's a finite capacity at any point in time. Torrenting is a bandwidth hog (and a lot of exit nodes will filter it out anyway)
Hey, we pay $100B/yr of tax money into the NSA/CIA/etc budgets every year so they can run exit nodes among other activities, I wouldn't exactly call it donated
But on a macro scale, the entire Tor network has fairly limited bandwidth and torrenting is a very easy way to saturate it. (Existential risk to the network / tragedy of the commons)
Probably going to be slower than over the Tor network without any manual tweaking.
...would also help with privacy and nasty telco letters.