The correct value is 5ns.
The LCD runs off 16-bit parallel with 16-bit colour so for lines or filled squares where the colour information is static, all that needs to change is the clock write cycle. The screen supports windowing so X and Y address counters are maintained by the LCD.
My plan is to combine a variable frequency multiplier with a duffs-device style rendering loop to limit the number of iterations used in each loop. For now a 2x multiplier will buy me enough CPU time to start testing other features.
The system is intended to drive vector animations for a game.