From my limited testing this issue is indeed real and multiplies itself by dupes and available tiles to reach.
To reproduce this:
Build a mid sized base in the center.
Wall it off.
Have two entrances: One with the old suits, one with jetpacks.
Make sure only one of those can be open at a time.
In my current game, at around cycle 600 with 20 dupes, i am running at 30ish fps with my GPU sitting at 15% and my CPU at 25-35%. Once i open the Jetpack exit, so dupes can only move out with a jetpack, my frames drop to as low as 3fps and average about 6fps.
If you dig out about everything on the map, the lag reduces again. This is likely due to dupes suddenly having access to WAY more potential stuff to do. Since neither GPU, nor CPU seem to run into any stress, id guess that this is down to some path finding algorithms that need some multithreading love. And yes, i know how much that sucks for path finding,... there are ways to make it kinda sane tho.