That crash was caused by some old code that I missed when I added the ability to configure the max depth of the world and just happened to not effect my test worlds. It has been fixed (I hope) in v0.4.1.
The FPS hit will always happen for a few minutes in a new world (or a world that Subterra is added to) until initial the chunk generation finishes. (On my laptop, it takes about one minute to calm down when max depth is set to the max of 4) This severity of the FPS hit will be proportional to the number of underground layers enabled (Configurable in the Mod Settings). Essentially, Subterra is generating a new "world" for each layer (called a "surface" in the API) and there are additional things that it does to the underground chunks, like remove all decoratives, ores, trees, etc. each which cost additional CPU cycles. So if all 4 underground layers are enabled then the game has to generate 5 times the chunks it normally does, and four of those chunks required additional processing.
If you add SubTerra to a world that already has many chunks generated you will see a FPS hit for much longer because it has more chunks to generate at that time than a new world.
That said, once the initial chunk generation is done, you should not notice a FPS hit unless you are exploring very very fast (which in the base game can only happen using commands or in sandbox).
I am exploring some methods to spread this initial workload out, we'll see how that goes.
