Really high.. I've only ever seen that happen on my MBP when a separate parallels service ending with "natd" was running, but that service itself was using 99%. After removing it from my startup items all was well.
I actually just ran into this.. Running vista business with dreamweaver open in the background.. my CPU is sitting at ~80% cpu usage while photoshop is focus'd. All parallels taking it up. Vista has the system idle process sitting at the usual 90+% cpu usage. While Parallels eats up everything in OS X. This is actually hampering my photoshop performance pretty heavily.. >:|
only time i saw my parallels doing that, was when my windows install suffered from the windows update bug that was causing svchost.exe to run at 99% cpu causing parallels to chew up all the cpu time it could. fixing that problem took it back to the manageable 20% it runs at now.
