My reply assumes you are using Elekscam.
I have never tried to do large grayscale pictures. But I have done large engravings, And I've notice that that Elekscam has an annoying habit of slowing down the feedrate after a few hours. I tried everything I could think of, changed the baud rate for the usb / serial interface. Raised the priority for the Elekscam processing thread. Shut down background tasks on the computer, Ran program as administrator....
The only thing that sped it back up was a clean reboot. and then it would slow down again hours into a burn.
What ended up working for me was Zak's T2 Laser. And I'm not sure if his software has fixed it, Using his 1.1 ROM fixed it, or if I just don't see it as much, because his program seems to do the burns much quicker.
I've also noticed the light streak between the eyes (just above the nose) follows the grain line in the wood. and that is because the the different shade of wood / grain density reacts differently with the laser. The only cure for that is a more consistently grained wood.