Hi setrone-
we've been struggling with aMule's CPU hungriness for a while now (primarily on Mac OS X). If I remember correctly, the whole thing started some time after 2.0.3. Ken and I have put a lot of work into trying to figure out what the cause of aMule's resource hunger is, using profiling tools such as Shark, but to no avail so far.
Usually, it takes a few hours until aMule max'es out the CPU and becomes slow. The main factor seems to be (as far as I can tell) the number of popular files you download.
One reason why the aMule-Mac.CVS.2006-01-14.Debug version may run a little better on your machine than the 2.1.2 version could be a modification I made to the core timer (I set it to 500ms instead of 100ms). On my machine that does not seem to fix the issue but it seems to postpone it a bit. I think, I will from now on build the release binaries with a slower core timer interval, too.
One thing you can do to reduce aMule's CPU usage a bit is to switch to a window that does not get updated frequently, like the messages or the search window. Whenever I'm not interacting with aMule, I switch to one of these windows and then hide aMule.