Well, I experimented a bit.
Yesterday I wrote that the game started to lag within a few minutes with and without mods.
Today, I repeated the experiment: I emptied the modlist.txt file, removed all current game.db and game_backup_*.db files and copied a backup batch of .db files that I archived in September to the /Saved directory.
I was rather surprised to discover that the game stopped lagging. That’s right, no FPS drop while running, no disappearing terrain, properly spawning NPCs in cities…
Right now, I’m adding back one by one the mods from my old modlist.txt to the emptied active modlist.txt and check if the issues return after adding a specific mod.
I’ve already tested Glass Constructions and More, Mining and Lumbermill, Pythagoras Support Beams, Extended Cartography, Improved Bodies, and Crassus. Crassus introduced a slight glitch: spawning near Sepermeru initially prevents NPCs in the city from spawning. I suspect that it has something to do with the fact that Crassus adds a “corpse” token to the NPCs’ loot table. Anyway, after running away from the city and returning there, the NPCs respawned and behaved normally. Crassus caused no lag.
I’ll continue my experiments. Yesterday I already noticed that adding back certain mods (mostly those related to NPCs or the player character) was certainly related to the lag problem and the disappearing terrain issue.