OK, so it's not backlash it's your motors losing steps.
Steppers have lower torque at higher speed so like I said above, slowing down would probably show improvement.
There are several possible causes.
You may have too much friction, either belts are over tight or the wheels/bearings are sticking.
Your motor current isn't sufficient.
The power supply isn't able to provide enough power to the laser and motors - this is common on the 5W and higher lasers. They should have their own dedicated supply.