I just ran a test on a 900K, 541 page PDF file, stored on both the 2GB card and Internal Memory.
I turned the first three pages in both documents, and turned three pages 90% of the way through the file. I could not detect an appreciable difference between the first pages and 90% though the work, nor between the Internal Memory and the 120X speed SD Card. All pages ran at about 1.6 seconds from button push to completed new page.