Hey ,
I noticed the same thing when I generate or update my voxel-world. I did some performance tracing and here are some rough numbers:
Generate mesh surface: 4ms
SetProceduralMeshTriangles: 0.2ms
Update Navmesh: 0.009ms
As you can see, the numbers show that an update of the mesh should not have that big of an impact on fps. In fact, it should not even be noticable. However, there is a very noticeable FPS drop and stutter in the game the moment after the update. Using the Stats tool in the engine provided little insight into what causes this. This max “Tick” time after the update is 100ms, but I have no idea where they come from. I suspect it to be some engine-internal stuff like adding the geometry to the world or the renderer.