I've got a P3 450, and PDFs cause me no troubles, in general. With every machine I've used, the only time I've seen slowdown is with more graphically intense PDF on a slower machine. In those cases I can see the page refresh slow down when I go scroll or zoom pages. But for normal stuff, there's no troubles. And just having one loaded doesn't has never caused me problems.
I don't know how your internet connection would affect things, except for the initial download time. After it's loaded, it's irrelevant.
As Mike said, there's something wrong with your machine or software installation. Have you tried reinstalling Acrobat?
EDIT:
I did a search on PDF related slowdowns and found some reviews at Download.com (
http://www.download.com/3302-2378_4-...html?pn=3&fb=0) echoing similar frustrations. I wonder if the newest version of Acrobat has some problems. I'm running v5 at home and it's fine. I've got 6 at work and it's ok too, but maybe v6 doesn't play well on all computers?