Paradox as a publisher seems to pull this shit a lot. Was really looking forward to Lamplighter's League, but the buggy mess that it released in shows very clearly that Paradox shoved it out the door before QA had some serious time with it.
They don't do this shit with their in-house games, seems to be an issue with them as a publisher.
Stellaris is like one of the most simulation heavy 4X games there is. There's a limit to how good performance you can get when you're making a complex simulation like that.
I haven't played Stellaria in a while, but I don't remember the performance being bad. If it is it's not because of any graphical thing that can be improved, but for the same reason Dwarf Fortress runs like shit, i.e. there's a limit to your CPU power when simulating hundreds of individuals.
The problem with Stellaris is that they simulate every individual even if nothing changed about them. If nothing changes on a planet or species then they shouldn't recalculate employed pops.
Hey, the Sims 4 modders, who use free 3d models to create badly made furniture mods, just to sell them for $20-30 on Patreon want to have a word with you