fei-d
That's quite strange behavior, the only reason I could imagine that would lead to something being slower is memory problems, or a bug in PRS+. Did you try to restart and check if that helps?
Did you try to disable MS SD scanning (an option in Settings -> PRS+ Settings -> Browse Folders) => copy book you want to read to Internal Memory (via Browse Folders) and open it from there? (I know, it's not as convenient as opening stuff directly on memory card, but it speeds up things a lot. Basically it allows you to have tens of thousands books on sd/ms card. Opening cards directly from external card with disabled scanning will probably be implemented, but not in the nearest future)
PS
PRS+ User Manual (by Mike_73):
http://wiki.prs-plus.googlecode.com/...Plus_10_en.pdf
check the "5.1 Browse Folders Settings" chapter.