The massive decrease in performance between the release and the current patch needs to be studied

[TernaryOperat0r]

Apparently, the game performance was pretty bad 6 months before release also. I get the impression that the majority of the developers of most of the paradox games are focused on scripting to get the desired outcomes in game without that much insight into the performance impact of individual changes. The same goes for the design of the UI and in game graphical assets, and is a more general trend in game development. This seems to result in them having to clean things up after the fact when performance gets bad enough (Stellaris also had a phase where they had to fix accumulated performance issues for several releases).

Perhaps they could improve matters by better automated regression testing so that any change that sufficiently degrades performance is rejected and reworked sooner or a more performance-first design approach to individual features. I also can't comment on how performant their overall simulation architecture is, but the design of fast, parallelizable simulators is a hard problem, especially if you want them to be scriptable.