Quote Originally Posted by Alex_M View Post
It is a Prusa I3 With the 5mm Threaded rod. I have...
#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 25}// (mm/sec)...
That value of 5 might be high, but like Mjolinor said, don't mess with the firmware yet. Find the setting for z-axis movement speed in the slicer and experiment with it. A number of factors come into play here, including the stepper driver current limit setting, the weight of the carriage that the z-motors would lift, acceleration, and moving up vs. moving down.