As far as i'm aware alterations to existing terrain have the biggest impact on fps. I no longer level the ground out and remove stones. I try to leave natural terrain untouched and build around it. I get around needing a moat or an earth wall, or anything really by not killing any bosses on this server.

If you press "F2" you can see how many instances are in a given area ( how many additional things the game must track). For me on an older machine i'm usually ok at 3-4k instances. When you start getting into 6-10k range I get big drops.

EDIT: well not really an old machine , its a 2020 i7, but its a SFF with a tiny psu and the only video card I could find that would fit the sff and the 130w ps was a 2 gb gt730.