[gnea/grbl Issue#727] Z axis not moving right after another axis stopped

I am most certainly sure, that this is an electronic problem, but there is a way to make a workaround in software, since the electronics of this project can’t be altered.

The problem: Z axis is not moving, right after one of the other axes stopped. So e.g. the mill moves it’s planar position to where it should drill, but Z won’t move downwards there.
BUT if there is only a fraction of a second delay in between stopping X/Y and moving Z, it moves without issues.
I already tried hard-coding a delay in grbl in various locations, only resulting in stutteringly motions, when milling circles.

My question to someone, who knows and understands the code better than i do: where does it make sense to put a hard-coded delay, so it will only delay right before a Z movement?