The scanning speed depends on the quality of the scanning module. The quality of the NEJE mini-module is only about 50g, which can provide the fastest speed.
Just so, the speed is stored in the GRBL and LightBurn can move only as fast as it is stored in the GRBL!
Apart from that, 66 mm / sec. would be damn fast!
So either something is set wrong or not right.
Does it make sense to move with 66 mm/min?
At least something can not really be lasered at this speed.
How about posting his GRBL settings because such questions can never be answered correctly if more information is missing!