That’s a good question.
The fastest material cutting speed listed at: http://docs.carbide3d.com/support/#tooling-support is 100" / min.
The original homing seek rate setting was 1000" / min., and then that was suggested to be 1500" / min for XLs and XXLS on the Grbl defaults page, and the default is now 2000" / min. I believe.
The thing is, the machine can only accelerate / decelerate so quickly.
http://docs.carbide3d.com/software-faq/shapeoko-3-default-grbl-settings/ has
$110=5000.000 (x max rate, mm/min)
$111=5000.000 (y max rate, mm/min)
$112=5000.000 (z max rate, mm/min)
$120=400.000 (x accel, mm/sec^2)
$121=400.000 (y accel, mm/sec^2)
$122=400.000 (z accel, mm/sec^2)
so one ought to be able to calculate the max speed the machine can get up to — might be that one could tweak those settings for performance — see “Shapeoko Racing League”: