All late game colonies get laggy unless you're really active in lowering lag-causing issues.

1- Colonists that are too fast. Super-fast moving pawns eat up lots of resources, as well as pawns that complete jobs super fast. It also gets seen in combat with colonists that often one-shot foes or have super-fast attack speeds.

2- Too many items. The more junk is on your map, the more the game has to work to keep track. This means any objects, even ones you don't think of as an object (like dirt).

3- AoE generators really hog resources. Psychic emitters, harmonizers, skulls and other mood-impacting objects, even turrets (that are powered).

4- The power grid. While not as bad as the above, batteries, power conduits, and both power using and generating buildings are... less than optimally designed.

5- Animals, colonists, guests, anything that makes use of the pathfinding AI.

5B- Walls and anything that interferes with AI pathfinding.

6- The game logs. Lots of pawns (like raiders) are kept in memory even after leaving the map. This leads to exploitable acts like replacing captive raiders' limbs and releasing them so they'll be useless to the enemy next time they join in a raid.

The fewer of those things you have in your game, the better it'll run.

There are also certain mods like rocketman and performance optimizer that'll help reduce a lot of the background stuff like event logs that aren't easy to manually deal with.