Try to overclock or underclock the emulator. The emulation speed depends on many things. The most important are:
The speed, type, FSB amount and cache amount of the CPU (the emulator runs faster on Athlon XP CPUs with "barton" core and Athlon 64 CPUs)
The speed and size of the RAM
The amount of anisotrofic filtering, antialiasing and resolution (slower/older videocards cannot handle high resolutions and high values of AA and AF)
The usage of "heavy" programs while running the emulator
Fragmentation of the hard disk
You may be able to fix the GFX errors if you enable the z-write options of the emulator.