I have AVRStudio 4.12 sp4 (build 498) installed on a Celeron 2GHz, 1G ram, WinXP pro. I know it isn't the highest spec computer out there but AVRStudio pulls between 20 and 40% cpu usage without even having a project open, let alone actually doing anything. To verify this I have killed all but critical processes and AVRStudio; the same thing happens. If you're debugging or something this peaks to about 70%. This is OK if it's the only program open but, for example, I was doing some AVR work in AVRStudio while compiling a GCC with AVR32 support (through Cygwin) in the background. Not only did the computer turn to a ball of crud, but the GCC build took 3 hours where it usually takes about 45mins to an hour.
If this is normal behavior, then it seems a bit wierd but I'll live with it. But that's the thing, is it normal behavior?