very gaming system dependant as well, especially if you are making mountains out of mole hills !!

Yeah, the terrain deformation performance hit has been solved for quite a while.

Turn Particle lights to Low, that's the biggest performance impact, especially in the mountains.

Do as less terraforming as possible.
also. The more meshes (objects) are whithin a certain area, the heavier the impact will be.

that's particularly true for bricks when building walls.
Although 2x1 Bricks look better, use 4x2 whenever possible.

Terrain editing with the hoe and pick shouldn't be that big of a burden on your fps

The exception would be if you're using mods to do large area shaping quickly, those mods still use the first iteration of how changes are saved, which was very taxing.

The current version is far more optimized and has very little impact on a decent set up, while on the other hand excessive fire/light sources, objects on the ground, or excessive npcs whether tamed or just present have a much higher impact.

There are examples in which the player had spawned in a ton of fish into a body of water and thus made it tank fps. Or just spawned hundreds of items and their system cant keep up.
Others have made large tree farms on slopes and just left logs piled up and the sheer volume of objects and the calculations that go into tree hp and movement tank the fps.

FPS tanks also happen in multiplayer situations because of how the peer2peer works and can get especially bad if someone has a poor connection or system, it will bog down the others.

Separate your builds on different biomes or different parts of large biomes. This helps a LOT if you're building multiple structures on one seed.

Also instances can play a role in FPS drops. So yes certain parts do cause more fps drops. Try to build where the instance line begins.

Press f2 and look at the instance count. The higher it is - the less frames you'll have. I have builds that go to 25000 and I barely get 15 frames there, so I'd recommend you don't go above 15-17k.