This is interesting.
There is some high-intensity code in the terrain materials building system. I was at one point going to move this to a compute shader because I thought it was too much for the CPU. However, once I had rewritten the algorithm to work with shaders, it was then running fast enough that I kept it on the CPU. (I actually had to write my own sorting code to make it work.)
There are two assert statements in the algo that did not have any messages. Simple adding some text in the second argument to Assert was enough to slow down the code to a crawl, which is what you saw. This is not normally something you need to worry about, but the Assert statements were being called more than a million times.
I will upload a new build after lunch. You are probably asleep right now.