If you're lucky enough to be running Aperture 3 under OS X 10.7 Lion, you may be irritated to find that since upgrading from Snow Leopard, Aperture has turned into some kind of memory-eating sloth. It has been irking me for some time, especially having to wait for 30 seconds between brush strokes when dodging and burning.
This issue only seems to affect Macs with 4GB of RAM or less. You'd think 4GB would be plenty, but apparently not. Even when the system has 800MB completely free and available, it still swaps Aperture in and out constantly. This is obviously less than optimal.
But help is at hand, and it's really simple: run Aperture in 32 bit mode. Functionality is completely unaffected, but it uses the pre-Lion memory model so runs just as well as it did under Snow Leopard. To switch it, open the Applications folder, click the Aperture 3 icon, and press ⌘-I. Then just check the "Open in 32-bit mode" checkbox. Sorted.