Hi,
what inductance are the steppers? Inductance is a good measure of how fast the machine will go before it starts missing steps. These cheap
machines tend to have high inductance steppers and are let down by them.
You may need to replace the steppers with low inductance ones, 1mH-2mH, 1mH preferred and reject anything over 2mH.
The second let down with cheap machines is the wimpy power supply, commonly 36VDC. To make your machine sit up and take notice you'd want at least 60VDC or 80VDC preferred,
but your drivers probably would not handle that, so you'd have to replace them as well.
Craig