"A couple of decades ago I bough a Sun Ultra-60 and asked whether it was worth paying for the dual processor version.
The answer that I got was "Yes. You need one processor to run X and another for everything else."
People cursed with diskless Sun 3/50's would run xterms and emacs's across the net on each other's workstations instead of locally. It was noticeably less sluggish, because you didn't get hit with all the heavyweight process context switching between X clients and server every keystroke.