First, just to let you know, I've just started using CS5 in the last 10 days.
I'm bringing my raw files into ACR, just one at a time and just doing global edits, white balance, exposure, recovery, fill light, black point, mid range brightness, clarity, vibrance, ect.
If I move one of the sliders, the little hour glass pop's up and there's about a 4 to 5 second delay before I see the result.
I'm not sure about this, but when I first start out I think its a little quicker, but by the time I start to edit the luminosity and saturation sliders, 5 sec wait for sure, and that's really a drag when your going back and forth between plus and minus side of one of the colors trying to find that sweet spot.
BTW, I'm opening Bridge from CS5 if that matters.
Also, if I use the magnifier in bridge, there's a longer wait for that to refresh (the image starts blurry, then clears up)