While iTunes itself is not a tremendous processing resource hog (about 1 to 3 percent CPU usage and 11 to 13 threads when idle), leaving the iTunes Store as your active selection in iTunes can jump the CPU usage to between 6 and 10 percent and add two to three threads when idle.
If you're needing to process anything requiring a lot of power and you've got iTunes open, being sure that the iTunes Store is not the active selection in the iTunes sidebar is a good place to get some extra oomph. Mac OS X Hints user scott721 writes:
After some experimentation I found that whenever the source (that list of items in the left-hand column) selected was the iTunes Store it caused a significant jump in usage. The only thing I can figure out is that the constant updating of the store's home page, revolving graphics, and other real-time functions are using processing power whereas keeping the source selected to something more benign, like 'Podcasts' or 'Music' that accesses only locally stored content (i.e., already on your hard drive) doesn't have to do that.
To check your usage statistics on your Mac, open Activity Monitor (Applications > Utilities) and check out the % CPU and Threads columns. You can sort the columns by clicking the column headings.
According to Mac OS X Hints moderator crarko, the Genius function in iTunes is another big processing hog. If you've got iTunes transmitting Genius information, try turning it off for some extra speed.