Assuming that you have an HISM in place, the engine has been slowing to a crawl anytime since .18 whevener the hism collection is selected and editable.
To work with them, I just run bluetility scripts and manipulate them via code once they bulge past the 1000 or so instances which the engine can work with.
Not sure if that is whats happening here, but it does sound similar…
Note: if the terrain/blocks are highlited in yellow, than thats the case.