z axis speed and marlin max speed

Hi,
in Marlin configuration file there are max speeds, what happens if the speed at slic3r (g-code) is higher than max speed in Marlin config file?will it miss steps?become slower?
I want to slow the speed of z axis because I've noticed it was not accurate (troubleshooting it), I set "speed for non-print moves" for lower values I suppose it will make it slower

Speed in firmware (e.g. Marlin) will prevail over speed set in slicer.
For example, if speed in Marlin is limited to 100 mm/s for x axis, and you set a maximum travel speed of 130 mm/s, travel will be performed at 100 mm/s.
You should first calibrate your printer so that the speed, acceleration and other mechanical settings work well.
Then the slicer software just make use of the printer capabilities, but cannot force it to go beyond.