See https://duet3d.com/wiki/Configuring_RepRapFirmware_for_a_Delta_printer#Homing_file, or read the comments in the sample homedelta.g file that I provide.
That F parameter is actually what I was thinking of when I said I didn't really want to increase the motor speed. However, its been many months since I last read that and had forgotten about the "300" values. I bet that's my issue. I'll double check that tonight. It's probably doing exactly what its been told to do. My Z height is somewhere around 335mm now…