Just got back from the Apple Store - with the App Store pricing for Aperture, I was tempted to get it and wanted to play around some with it. Here's what I noticed...
13.3" base MBA - launching Aperture is lightning quick. Loading up photos (loading on display) because they are all in RAW in the sample projects, does have a lag. Making adjustments for RAW (eg. sharpening) is noticeable.
15" MBP i7 - launching Aperture is dog slow (why does Apple even bother with those lumbering 5400rpm drives anymore?). Loading up photos is a lot better (to get the loading to go away), and RAW adjustments is much quicker but still not instant.
27" iMac - launching Aperture is not bad (not SSD quick, but nothing like the slowness of the HDD-based MBP). Again, RAW adjustments can still take a second or two to process like the MBP.
And the default sample library the install for the store displays isn't too large. I image a huge Aperture library on a slow drive is going to be a nightmare to deal with. Hence the SSD is actually a huge boost in getting the photos into memory, but the CPU will matter when working with RAW images.
Not a biggie for me - haven't really been taking photography seriously for a long while thus no RAW shooting, but I imagine a working pro photog would want more CPU muscle than any MBA can muster, though some light editing is certainly more than possible.