Title is the question.
If I set collisionfidelity to Box on a CSG or MeshPart, will the lag that the physics create be the same as the lag from a normal brick?
From what I know, setting CollisionFidelty to “Box” does optimize performance. It might be actually a bit more memory consuming due to usually more complex shape, so it probably won’t be exactly the the same as normal Part.
The calculation will be the same for both as they are both box hulls, the only variation there should be may come from a statement checking the hull type.
One way to test this is to create a place with 100 meshparts, a place with 100 normal parts, and a place with 100 boolean parts, all of which being equaly sized and shaped. Then, check the stress on the physics via that one setting. 
That one setting:
https://gyazo.com/e3fac5c873d4576d8dfcaaeed3a0ca91
The thing ur checking:
https://gyazo.com/a42b4d1b188dc1342046b3b1e6ac851b