Draw selections and other special graphics using transparency effects. Disable the option to draw them using outline or opaque fill styles and increase speed on selections.
Use a background thread to generate the pages. By disabling this option the GUI will become less reactive (will be blocked if necessary), but pages will be displayed a bit faster.
KPDF can achieve best performance by tuning the memory usage, based on your system and your tastes. The more memory you let it to use, the faster the program will behave. The Default profile is good for every system, but you can prevent KPDF from using more memory than necessary by selecting the Low profile, or let it get the most out of your system using Aggressive.