Resonance is why they stall on the bench but work fine on machine. With cheap Analog drives like those from Longs then it's worse than with better quality drives and much worse than Digital drives.

If those drives where Digital then on machine you'd reach 7-8mtr/min and have much smoother performance.

If your only getting 1.5Mtr/min on machine and sure all your tuning/setup is correct then I'd start looking for some serious binding.?

How did you wire the Motors parallel or Series.?

Why the Voltage isn't displaying I'm not sure it could be the plug-in or PC. Make sure your running Mach3 as Admin.