Exactly, it is my belief that a slow card can create a bottleneck to processing the file. I'm reasonably certain that this 120X ultra card (used to use it in may camera before I got a 4GB for it) is much faster (over-kill?) than the Reader itself.
I've been surprised that other users have had such slow page turns on even a 100 page document. In addition to card speed this may also be due to the file size, and the resulting size of each page. I always use the "Smallest File Size" option when creating a PDF file, which results in a file that is about 25% the size of a fully optioned file from Acrobat Pro 7.0