The default is in mm/s and is 150, which is the maximum the machine is capable of moving without skipping steps. Also that is screamin fast and not appropriate for a laser work speed. I’m not 100% sure what you’re trying to do but M203 is not the right place to be changing settings.
You are probably having the same issue as this: Snapmaker 2 Dwell setting Gcode G4:Px and Laser Speed