Is there a setting in Grbl Panel to limit the speed of the motors?
I did change $100 through $122 in the Grbl settings, but when I closed the program all of the values reset back to their defaults.

I've found that with my Nema23 motors (which have no identifiable marks on them) that the faster speed I can use is:

Code:
G91 G21 G01 X5 F35
At this speed, this is the fastest it can go with enough torque to not have any slippage.


With some sample code from Fusion360, I manually set F to 50, but it accelerates so much that the motor just sits and vibrates back and forth.
Code:
G90 G94
G17
G21
G28 G91 Z0
G90


(Circular1)
T1
S12000 M3
G54
M8
G0 X-46.25 Y-0.5
Z15
G1 Z0 F50
Z-1.5
G18 G2 X-45.75 Z-2 I0.5 K0
G1 X-45.5
G17 G3 X-45 Y0 I0 J0.5
G2 X45 Y0 I45 J0
X-45 Y0 I-45 J0
G3 X-45.5 Y0.5 I-0.5 J0
G1 X-45.75
G18 G3 X-46.25 Z-1.5 I0 K0.5
G1 Z15
G17
M9
G28 G91 Z0
G90
G28 G91 X0 Y0
G90
M5
M30