U8glib: Graphics Lib for LCDs and OLEDs

I'm using the library for the DFRobot ST7920 128 X 64 LCD and it works without a problem, both in serial as in parallel mode. But the weird thing is, is that the parallel mode is a bit slower. In SW serial I got a refresh rate of 6 to 7 frames per sec and in 8bit it goes down to 4 to 5.

Is this normal behavior or am I doing something wrong?