We have been going over this for quite a while on the Steam forums. The devs have not really tried explaining what the problem might be, they might not even know. But the AIs and such only spawn around where you are. You can set the population in the city down in the options though, and it should help on the performance. For me it did not seem to do much one way or the other though.

I am pretty sure you could send your dudes across two separate districts and the game would still render entire areas along with every single NPC from what I've seen.

Seeing as this is a Unity game couldn't someone decompile the game, disable the NPC spawns, reduce the enemy spawns to like 1 or something and check if the performance improves? Maybe even disable the AI but it would probably be easier to just set the spawns to 0.

I wonder if the fog of war could also be disabled in order to check what else the game does. I wouldn't mind giving this a shot myself but I'm not experienced enough with these things.

All of that should be possible, even without decompiling. Its possible to mod the game now, and I am pretty sure that using this I should be able to chose to just delete all NPCs, I guess it might also delete all enemies, but there should be ways around that, and there should be spawn points, which can be deleted as well.

Been a while since I have modded the game, I recently tried changing the rendering mode on the games main camera, but it did not seem to help. But the in game option to reduce the population in the city should help give an indication of whether it would help to remove all the NPCs.

I'm absolutely certain that guards and civilians and such are removed from the game when they go far enough into the Fog of War. I've done a few experiments which have shown that, such as selecting a Dracogenics unit going across a district, and the selection then went away some distance into the Fog of War, and the unit never reached its destination.

Now, units sent out from spawn points to respond to alerts are an exception to this. They'll run through the Fog of War until they reach their destination. One time an Eternals patrol ran all the way to just north of the Ronin compound after my agents had been spotted by cameras on the way out of The House of Eternals side-door (the one with the turret).

I'm also fairly certain that only areas where the player has eyes (agents, hijacked, escorted VIPs) are being rendered. Sometimes I get the invisible tile bug, where parts of the ground become completely transparent. And that is usually when I have spread out my agents across several different relay beacons. If every part of the city was being rendered all the time, I'm pretty sure I'd get that bug more often.

One thing that seems to slow down the game a lot is fragmentation grenades. I can play for hours without significant slowdowns, but not so when I lob grenades all the time.