Performance leakage?

Has anyone else noticed a gradual "leakage" of graphics performance with longer play times?

I don't mean longer through a run with more complex villages, but firing up the game and then maybe 2-3 hrs into a session.

I thought it was just random stuff, but since I suspected it I've tried to keep an eye on it. I set the graphic settings to cap at 120 fps, and hit this at the start, but over time this drops progressively so that maybe 2 hours game play in I'm getting maybe 50 fps. I've seen the same behaviour on the last 4-5 game sessions in the same run.

Could be my hardware obvs, but I'm not really seeing the same sort of thing with Anno 1800 that I've also been playing recently, which is somewhat similar in gameplay/graphics.