Selecting multiple cores is more likely to crash Ubox, that happened when I tried to use it. I agree that multicore would be extremely helpful for this, but Dan wasn't able to get mulltithreading (which is multicore) to work for some reason or other.
As I said, the next major release sometime this year WILL have multicore capability.
As for your testing, the slow drop sounds like trails, did you hide them? Also toggle light mode (old/new) and bodies (alt+b) to see how that helps you. I don't think turning on or off the background has any effect, also if you have lots of named objects, turning off labels could help (not to mention remove visual clutter).
Edit: checked with FPS on and turning the background on and off does seem to affect it, apparently.