DRV8825 on ramps 1.4 z axis

This is my very first post on this forum, hopefully you great people will sort my problem.

In my setup I have two nema 17 0.8A stepper motors, Pololu DRV8825 and ramps 1.4 on mega 2560. all the axis are working fine, but z axis is moving very slowly, is it because I have installed dual motors on single driver or there is some thing else that I am missing ?

Value on VRef is 0.40 V


Could you be more clear about what you mean by moving slowly? Is the movement smooth? Posting a video of the movement here might help clarify the problem. Can you also post pictures that show the DRV8825 board and how you have the motors connected to it? What did you set the VREF to?