As I noted a few days ago I upgraded to Aperture 3.1, and I also just upgraded my MacBook Pro to 8GBs of RAM. Even before I upgraded the RAM I noticed that Aperture 3.1 was snappier than earlier versions. The speed of exporting images has been improved, and I'd say the the transition between libraries has also been improved as well. Adjustments seem to be applied faster, whereas under 3.0.3 and earlier there was a longer delay.
The memory leak/bug that I have talked about in the past, that occurs when you work with brushes, is still present. I was working on an image, brushing in some curve adjustments, and 4GBs of 8GBs of RAM was free, suddenly it was all gone and Aperture 3 was gobbling up swap space like nobodies business (over 1GB when I force quit the app). When I first noticed this bug back in version 3.0 it was much worse, occurring any time I worked with brushes. Now it doesn't happen anywhere near as much, I had worked on four other images, making adjustments with brushes, and had no problems at all.
The speed of exporting images has been improved greatly, taking at least half as long in Aperture 3.1 as under 3.0.3. That is a very welcome improvement, especially when you are trying to upload a large group of images. The speed of "loading" images is the same, I'm guessing that it is more hard drive dependent than the program. I have noticed any difference in import speed, most likely for the same reason as noted above.
Overall I'd say that version 3.1 is a big improvement, but the memory leak/bug that I have talked about in the past is still present, which is disappointing. I really hoped that version 3.1 would nick that bug/leak, but that is not the case.