Yes I can imagine. I am just evaluation depending on the worst case scenario.
After reading a bit more about how instructions and clock cycles, I learned that a clock cycle is not just 1 bit but the size of the MCUs bus (in my case 32 bits)
So I guess writing to PORTB which is 16 pins, equals 1 instruction. So since my LCD has a 16bits bus, I can write 2 bytes in 1 instruction.
Meaning that 1 pixel is 1 instruction, I would need 76800 px/frame X 20 frames/sec=1.536MHz like barry said.
Am I right now ?
I really appreciate your help.