I had aMule peg as much CPU as it could get when I was downloading just one file at 100-200K and uploading at 50K. I was also just sharing this one file. Less than 100 sources total
But aMule has always been a resource hog, so I don't think it's anything with your build.
I think there are great optimization opportunities, I tried sampling it in Instruments, and there were 3 things that took 75% of the total. There were also some unicode functions which took 5%, which I find very strange, as it shouldn't need to convert unicode all the time.
Note I'm not a programmer, so my knowledge is not the best about this.