Speed was my top concern last spring when I was debating whether to adopt Apple's Aperture or Adobe's Lightroom as my main photo tool. One of the reasons I ultimately picked Lightroom was that it did not demand the latest, fastest chips to run smoothly. In contrast, reviewers' tests back then found that Aperture had a major CPU appetite. In fact, my one-year-old Macbook wasn't fast enough—and it was my fastest Mac machine.
So I passed on Aperture. (The other big reason being that I find Aperture's controls quite awkward, in the same way that its little cousin, iPhoto, feels like a computer tool instead of a photo tool.)
In his review of the just-released Aperture 2, Fraser Speirs says the update's speed is now on par with Lightroom. Although I'm now wedded to Lightroom, his take and stats are worth a look.