[B18] Colonists slow down going through held open double doors

[YokoZar]

I setup a lot of double doors, both held open.  One wood, one limestone, both with constructed floors on them.

I notice the colonists slow down when passing through the limestone part of the doorway, for no apparent reason.

[YokoZar]

Some further investigation and this happens with all double doors -- double wooden auto, double held open, whatever.