Are you rendering at the same sample rate as you're recording? And similarly, is your soundcard set to play back at 44.1k as well? If you have to do any resampling when playing, then the CPU will take a hit, however just playing back shouldn't be very CPU taxing at all...
My soundcard accidentally set itself to 192khz while in my bag - then experienced really crappy performance and took me a while to work out why...
As for using all your processors - I think Reaper will work with what it's given - I know it uses all the cores by default when using, say, a quad core processor but I'm not sure about physical CPUs. It probably does as it's nice like that.
Never gone anywhere near UAD.