Not satisfied with the answers given here https://forum.arduino.cc/index.php?topic=626707 ?
Well, configuring GRBL to play nice with your machine is your job to do. If you configure it to be slow, then it will be slow. If you configure it to work at optimal speed with your CNC, then it will be fast. Nobody to blame here but the user Don't know which genius you payed to write the code (and which code that would be, as you need a PC program and an arduino program), but you get what you pay for. As you do not want to use proven working code you will most likely end up haveing spent way more than the 200+350 bucks for nothing.
g1 x0 y0g1 x50 y50