GRBL Controller - Inconsistant speed on image engrave

Im just learning LightBurn and am using the trial to test out my new diode laser I have attached to my GRBL CNC machine. Im running a photo engrave at 2500mm/min (changed the default speed from mm/sec) at 100% power at 350DPI. I can see that the stepper motors arn’t running at the consistant speed of 2500mm/min, it is stuttery in some parts of the image. From doing a bit of research, I’ve come to the conclusion that my GRBL controller can’t keep up with the amount of instructions being sent and therefore slows down/reduces power to catch up. Where there should be a smooth gradient in the image, the laser burn is inconsistent and jittery. Am i expecting too much of my GRBL controller?

This is thr GRBL controller I have :
The store told me
“The 8-bit ATmega328P on the Arduino boards runs at 16MHz – that’s 16 million cycles per second.”
Looks like that’s the same chip thats on the Eleksmaker.

What kind of max speed/DPI should I be expecting out of this card. On another forum post, they said 8-bit GRBL controllers only handle about 400 instructions per second. How can I calculate the maximum Speed vs DPI I can have? And also if I was to upgrade to a higher performance card, what should I be looking at, keeping in mind I do CNC routing as well as Laser on the same machine.

It’s also 1/6th clock speed and 1/4 of the data word width of a smoothie board or the same processor used on most laser diode systems (Ortur, NEJE, etc).

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