In a word, SLOW. I'm running a multi-core W/XP system (with support for 4+ "CPUs") and Reader, using files off my hard drive (as opposed to Network or Internet) is slow. It is so bad that I would go back to 9 if my employer gave me the ability to do so. I keep my drive defragged, I have 4GB of RAM (which W/XP 32 only uses a bit over 3G of it).
But ever since the upgrade to X has been rolled out to us (read that, jammed down our throats), Reader has been slow. I can't even move it from one display to a second without it having to stop and catch its breath to finish moving (yeah, that's exactly what it feels like).
Meanwhile, on my SuSE 11.4 (32 & 64 bit) systems at home, I do not see this slowness. In fact, W/XP in VMware running Reader 9 is faster than this!! And that system is constrained to 2GB and 1 CPU.
Edit > Preferences > General and deselecting Enable Protected Mode at startup
Here is some detail on Protected mode: http://kb2.adobe.com/cps/860/cpsid_86063.html
Kelly McCathran
Adobe Community Expert
Certified Technical Trainer+