To achieve a refresh rate of 60 Hz (progressive scan), we need to clock 60 * 19,200 = 1,152,000 nibbles per second. In this design, each nibble takes 4 cycles, which includes the DOTCLK toggling. There is also about 70 cycles overhead for each line (register setup, Hsync, sometimes Vsync, Enable). The total number of cycles required to refresh this display is thus 4 * 1,152,000 + 70 * (240 * 60) = 5,616,000 per second. For an AVR running at 20 Mhz, this means it's busy 28% of the time doing refreshes, leaving over 70% for other tasks, mainly to update the display memory based on external graphics commands from the user interface.