After completing "Explore the ruined tower" quest, in particular - after returning to the city, frame rate and responsiveness dropped pretty hard. I opened task manager and saw that Solasta was using over 6GB RAM. Creating a memory dump took a couple of minutes, after which Solasta was taking almost 10GB RAM.
Not sure if it'll be useful, but here is that memory dump: https://drive.google.com/file/d/1ibZAb4OAFDjuZBvvtoclxHUo3aZkJb6S/view?usp=sharing
This was my second playthrough, with game version 2.8.0. I think I had it twice during my first playthrough (not at the same place), at version 2.6.0, but just restarted the game without paying much attention.
System info: Windows 7 64 bit, Intel Core i5-4570, 16GB DDR3 RAM, NVidia GeForce GTX 1070.