Forget the advice. Lightroom is faster, as noted in every review of both programs. Try Aperture yourself with the demo to find out.
Working with 10+ megapixel images is always going to be slow, and with camera advances, it will get worse every time your wife gets a higher resolution camera --so comparing it with how it used to be when you have 6mp files is not exactly correct.
More memory and an SSD will definitely help.