I finally got the ported code running and…
it runs like shit. About 15 frames per second, down from about 45 from the old version.
Well, there are a number of bugs left, so obviously I need to fix those up before I declare the sky to be falling. I will be seriously annoyed if this turns out to be more inefficient than the AS3 version, but the difference is, because I’m working with something that compiles down to machine code, I actually have some recourse if that ends up being the case. If I have to, I can write my own C++ rendering module to draw the game images as rapidly as necessary.
Nothing to do but get into it and see what happens. At least it runs now.