Any two double doors, even wooden autodoors or held open doors, will cause the colonist to slow down as they pass through the second one. It's as though they're stepping up on something with a path cost.
Been there since at least B18: https://ludeon.com/forums/index.php?topic=37327.0
Also Reddit thread: https://www.reddit.com/r/RimWorld/comments/7spjk0/2_or_more_consecutive_doors_will_slow_down/
This bug is still in the current version of the game (1.2.2900)
I have it written down, thanks for the bug report!