That is a vanilla game issue. I suggest playing a smaller map if you can.

Make sure you have enough RAM so your machine doesn't need to start digging into swap memory.

The number of pops is still the biggest contributor (I think?) so you can have a big map etc. but you have to keep the number of habitable planets and pop growth lower. It's all dependent on your system performance but for example I have quite an old machine now and I am able to enjoy busy galaxies as long as pop growth and habitable worlds are very low.

Pops is key. Play with the minimum amount of planets setting. That helped me a lot without reducing the amount of AI empires. Heck, even I bought a 3k pc last year i still can't finish the largest map this mod has to offer with max empires.