Purely guessing, but: gigantic generated funcs are A Thing™, and pretty often I'd expect that you don't even need to know the full contents of a func to being executing it. Some structures would require it, sure, but not all. You could treat it as speculative execution up to the point where you can't, and then you just wait and continue-or-error when you know more.
Author here: you got here before me but this is precisely the right answer, although it’s more about not needing the full contents to start _compiling_, rather than executing necessarily. Supporting streaming execution parallel to compilation is absolutely possible, but a tougher challenge that isn’t necessarily for our purposes.