The problem solved itself but I wanted to report this anyway. Maybe it's useful information for @bastecklein.
So my colony is pretty big, about 900k colonists on 460×460. I'm playing on the web version and haven't had any performance issues so far. Anyway, yesterday I left the game idle to finish building some 800 aluminum generators and 200 ultra deep dig sites, like I did many times before. After an hour or so I came back to check if everything was in order.
Except it wasn't. The game freezed every 2 seconds for about 1 second. Rovers had finished building the aluminum generators but had not yet started with the dig sites. They were stuck on "Moving" but did not move a tile. I selected all bots and instructed them to move to the dig sites. They lagged their way there and started building, slowly. I suspected the bug might have something to do with the construction sites or a path finding issue. So I just let the game idle for another 2+ hours to finish building.
When every building was finished and all bots were idle again, the lag and freezes stopped completely. Performance as good as ever, but I have no idea.
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I am experiencing this very thing right now.
150x150 Zolarg map (overrun with sugarcane). 1300 population.