I had 5.1 installed, and it was excruciatingly slow. 15 seconds to redraw a one page pdf ~ 420KB. I said redraw, as in if you scrolled all the way to the bottom. HW config: 2xP3-900, 1GB, Radeon 32MB (original). Switching to a Radeon 9000 w/64MB made no difference. You'd think the stupid thing would be in the veido card's RAM already! The m/b has a 1x/2x AGP slot.
I uninstalled 5.1 and installed 5.05 - blazingly fast. With some pdfs I do get a whine window that newer features cannot be displayed.
Along with jtr, I'm probably using the slowest system here, so nobody else will probably feel the pain as much as I did.