Yeah, that is weird, I'd wonder if you're missing something stats-wise or if your site has some Safari issue you're not aware of? I work with a range of different sites, and I don't think I've seen Safari usage under five percent for any of them any time this year. FWIW, I haven't seen the giant drop the OP is referring to, just a continuation of the slow slide.

Aperture's performance is fine for me, same hardware (MacBook Pro, 2.16 C2D, 2G RAM, 128MB graphics card). The (very individual) deciding factor for me was the interface- Aperture makes sense to me. Lightroom 's interface just isn't intuitive for my brain, I experimented with the beta versions as they came out and it was much more work to get it to Do What I Want.