I'm not saying that Stellaris's AI is the best it could possibly be. I'm saying that good AI requires trade-offs. Trade-offs that Stellaris's AI coder would love to make but can't because the game would be an unbearable slog if they did. The dude isn't incompetent or lazy. Just under difficult constraints and trying to handle multiple mechanic changes on top of that.
Stellaris is a different game than EU4, CK2, Victoria 2 and all of the other Paradox games. You can't import the same AI and get the same results, especially with the much larger emphasis on expansion and resource management that Stellaris has. The AI in CK2 just has to wake up from time to time, check its options, and pick which one it likes best. The AI in EU4 just has to keep itself from imploding while gobbling up provinces and the mechanics ensure that it'll be a reasonable opponent. The AI in Stellaris has to build an Empire from scratch and keep pace with a human player doing the same thing. If CK2 and EU4 have more and better AI, it's probably because less is demanded of the AI in those games. Not to mention their longer and more stable development.
Also, being pendantic, if those games ran more calculations per tick then they'd be running slower too. Assuming that the bottlenecks are the same or similar, a CPU can only handle so many calculations per second. No amount of programming voodoo will squeeze more calculations out of it. To run faster all you can do is reduce the number of calculations done per tick, which requires tons of optimization or a dumber AI.