Bittorrent may be an exception to the above but the performance cost would be at the clients end and for one bittorrent is hardly a realtime protocol a little delay making each connection would not make much difference, two it performs poorly if you insist on running it over tor anyway and thirdly the average consumer desktop system is not exactly lacking spare CPU cycles due to the bursty nature of their workloads they statistically tend to spend the majority of their time idling.
As opposed to torrenting via exits, I'm amazed more people haven't moved their torrenting to operate entirely within anonymous networks. The performance of I2P and Tor (and I guess Phantom during tests) is actually quite usable so long as the user excercises content selection and patience. And regarding this thread, the freedom of operators and users from complaints regarding takedown would be invaluable. I'm not sure if these networks could scale to handle the node count and chatter, but I think the bit about spare cycles is right and might even act as a natural limiter to that sort of traffic. For instance, my tests show that establishing connections to many onions in parallel is quite costly and prone to failure without available headroom. But once connected, things are ok. Perhaps torrenters simply can't put aside their 'must download it all right now' mentality which keeps them away from our nodes. Or there is a major influx waiting to happen upon some future enlightenment whether they're misguided or not.