Compressing DVDs is easiest with Toast. Still ages though.
Video can be monstrous to any CPU. I acquired (cough) the Matrix Trilogy in HD wmv format. My dual 1.42 MDD couldn't play them watchably, so we opted to try a conversion. I don't recall if it was MPEG Streamclip or Compressor but a dual-core G5 2.3GHz sat there all day from 9-5.30 and didn't get halfway through converting the first film. I ought to try on a Mac Pro. I forget exactly how big those files are, I think they were 20GB each or something stupid. I have a 12GB version of Pirates of the Caribbean but the MDD plays it fine becuase its not a stupid wmv.