This has been bugging me too, but a little websearch revealed this nifty little unix command: nice. (I found it on an apple discussion page). All you need to do is load up the terminal and type
Code:
nice -n 18 /Applications/Avernum\ 6\ ƒ/Avernum\ 6\ v1.0.app/Contents/MacOS/Avernum\ 6
or whatever the path to your program might be. If the path to the program is too cumbersome to type, just type the command and then drag and drop the program (note that you have to dig into the package contents) onto the terminal window. For me and my new macbook pro, this cut CPU usage from 100% down to 10% with no noticeable drop in performance.
[edit: drat. Didn't work. This will make it so that if you run the game in a window and go to another application then Avernum will only use a small percentage of the CPU (which it didn't do before), but once you switch back to the game it will be back at 100%. Oh well, at least I learned a new Unix command. [\edit]