Does it persist between restarts? Because that sounds like a memory leak to me.
Performance in general in saves definitely decreases over time because of everything new happening and appearing around the map, but it never gets super bad compared to a brand new save.
Viewing post in "Voices of the Void" Pre-Alpha 0.8.2b comments
I used to have an issue like this, the longer i would play the worse it got. Eventually i had to ask for help on the discord from someone who could no-clip. Turns out i had a few hundred roaches in the vents next to the chair! Once I got rid of them everything went back to normal. Don't know if this will help with your problem but i hope it does.