I feel like the game def has a memory leak/problem that sneaks up the longer you leave it running. If I leave it paused for some hours, then come back everything becomes totally unresponsive and eventually finally crashes. Even when I just play for a few hours it starts to get noticeably choppy.
I've been noticing this too. Odd problems creep up but go away if I restart the game. I have the rgb lights on my system set to change color with cpu usage. When playing Planet Zoo they are always blue or green for the first while which signifies low demand. Then after a few hours they are solid red, meaning the cpu is working much harder. Not much makes my i7-9700 go into the red.