Stellaris updates are slowing down to stop empires randomly destroying themselves, among other bugs

Like every major release, they redid some management elements, so people are unhappy about that. But the big thing was that 4.0 was a patch intended to address performance with how pops were handled, so the mid-late game was playable at a good speed. Instead the update made performance issues worse, and apparently even with a bunch of hotfixes performance is still bad, and there's a lot of issues caused by the patch other than performance.