It may simply be you don't have enough torque . It may be you have noise. It may be you don't have the correct current setting on your drivers .
Direct Drive Pinion setups (Pinion on the motor shaft) offers high speed rapids but lower torque. A better match is to use a belt reduction unit between the motor and pinion of from 3:1 up to 5:1 .
While pure weight affects acceleration its probably more the cutting forces that are causing the issue. Belts and pulleys can get you those ratios. CNCRouterParts sells some lower cost belt reduction units for rack & pinion. You may need to change out the rack pitch because the pinion of the belt reduction units is a 20 TPI 20 deg pressure angle gear.
You should start out by making an "Air cut" above the material (or rig a pin up on the end of the router spindle) and have it trace out the cut at normal cutting speeds but not cutting . That will tell you if its something in the electronics or a lack of torque.
Another issue may be noise from the spindle. If it uses a VFD converter/speed control the VFD noise is high and may be getting into the drivers. If you do not see devation in the "dry cut" test , try it with the spindle on but still not cutting..
Keep the VFD and the power wires to and from it as far away from the control electronics as you can.