I recently purchased a 27" iMac with the Core i7 and 8GB of RAM. I'm running Aperture 3 on it and expected it to be pretty blazing fast. It's quick enough doing most things, but I've noticed it recently slowing down a lot using some of the brushes tools. Has anybody else experienced this? Are there any particularly good ways I can test the performance of my machine to check whether it's running as fast as it should be?