The question was WHY do the axes on lasers move differently when they don’t on a CNC router, especially when the difference in mass is much more pronounced on the router?
To be more “brilliant”, there is a difference between speed and acceleration. You can have the same/different speed or acceleration on each axis.
Speed is actually velocity; how fast something is moving.
Acceleration is how quickly you get from current speed to desired speed.
Consider a sports car, pickup truck, and tractor/trailer (“semi truck”), each governed to no more than 60Mph (it’s OK, this is an analogy, not real life :)). They all have a top speed of 60Mph but the sports car will get up to that speed quicker than the pickup which will be quicker than the tractor/trailer.
All axes on the CNC may be the same speed, but they would not be the same acceleration (unless the axes capable of more acceleration are artificially limited).
The acceleration may be different, but quick enough that you don’t see the ramp up/down and just see them moving at the same speed. Or, the speed is intentionally limited.
I’ve only used a 6040 CNC with a UCCNC controller. It, as well as Mach3 and others, allow you to set acceleration and speed on each axis independently.
There may also be an additional design consideration; lasers just need to move stuff around. CNC has the additional load of milling and mechanical resistance.