Ill try to list some though I may be incorrect on some 
- Large mass of parts in one area being rendered
- Changing properties/rendering things such as material/color/transparency increase lag.
- Objects that are transparent between 1 and 0 (Ex: 0.9, 0.99, 0.01, 0.05) cause more stress though might not be that noticable
- Meshes/any unions that contain difficult rendering problems/collisions (If you’re using the default collision fidelity thing) increase lag.
- Collisions on unnecessary objects (Depends)
- Memory leaks due to scripts and connections that never get disconnected and keep stacking.
- Server sided calculations (Dont do tweens on server unless its super necessary. If you need to, use TweenServiceV2)
- A massive amount of cloning/instancing.new in a short span of time.
- An abundant amount of Heartbeat/Renderstepped in unnecessary spots.
- A massive amount of terrarin/land with parts. (Especially noticeable the more you go away from the 0,0,0 origin)
- Textures on sides of objects that are never seen. (Ex: Dirt texture below path that nobody will see below)
- Terribly unoptimized code/deprecated things.
- The use of welds/unanchored bricks in a massive amount.
There are much more but thats the jist of it.