The value that limits the z-axis speed is found in Settings.cpp of the firmware, on line ~86 - maxZRPM. It is currently declared as 12.6.
You can try different values. Be advised, this setting may have strange results in gcode files that do coordinated x-y-z movements 
The easy way to play with this setting is to program one of the GC macros to send the value.
$18=12.6
would send the default value. With a meter on the bench,
$18=20.0 seemed to cause the drive voltage to maximum, higher values didn’t seem to make a difference.
My sequence was to change the macro, use the macro button to send it, use the Z-axis panel to test it.
Let us know what your find 
Once you settle on a $18 value, you can try values for F (feedrate) in your gcode file as @dlang suggests.