It comes down to machine rigidity and material removal rates (MRR)
With higher rigidity on can go deeper per pass therefore increasing the MRR rate. The other benefit is that you dont just use the cutter tip only, but also more of the cutter lenth (height of bit) With proper chip load / chip thickness the bits last longer.
IIRC the Xcarve is 25mm/motor turn, the ACME is 8mm/motor turn so there is an increase in resolution/accuracy/torque, traded for all out speed.
Xcarve default rapids are 8000mm/min, with ACME you can expect 2500-4000mm/min on the same power system.