The critical speed for 1610 should be about 1000rpm (using a root diameter of 14mm) and although the machine runs nicely at about 4m/min cutting speed, the motors stall every now and again if doing rapids at more than 4m/min.
On my router the screws are 1540mm x 2 with nema 23 motors driven by pulleys and belts. I can get 8-10 mtr/min

It would help if you could up your power supply to about 70V if your drivers can handle that. 40V is a a bit on the low side.