So there are two Blueprint types in the pack:
This one creates dynamic geometry which conforms to static meshes around it. The geometry is created in the construction script, so at runtime there is no more overhead than if you had exported your scene to a 3D package, created a conforming mesh with UVs, then reimported it as a static mesh.
On the downside, it does not have static lighting capability (i.e. no lightmaps, only dynamic lighting), since it is a dynamic element. However this is no different than any other geometry that can potentially move (such as the door in the video).
In terms of render speed it depends on how many subdivisions you have (it goes from two triangles (making one quad) up to thousands of triangles for a 64x64 grid). I’ve not seen any slowdown personally on rendering, but if you have a very dense mesh, it can be sluggish to move around (not at runtime, but when you are placing it, since it re-runs the construction script every time you move it).
If you need a dense mesh, then it’s best to have a low res mesh (10x10) until you have placed it and tweaked the other settings. Then crank it up once it’s in place, after that it’s all pretty efficient. However you can often get the result you need with a relatively low subdivision level and slightly increase surface offset.
These are essentially decals with the convenience of the auto alignment/sizing code from TeraTransfers.
They both have advantages and disadvantages (some of which I mentioned in the video).