Right. Avast may just not be the culprit here.
I've now managed to create one such folder without the presence of the fake .exe, and still the slowdown on first open.
And I've opened one such folder with all Avast shields disabled. Also still the slowdown.
It feels like the folders open slightly faster, maybe, but that's it. Maybe also not.
There isn't much of any other similarity between the folders that open slow. Most have just a few files in them and those are different file types. It's also hard to test further because sometimes a get a lot of them and other times it takes days.
This leaves me with Windows / Explorer itself as the most likely candidate, unless Avast just straight up ignores exceptions and shield deactivation (and I know it at least does the first).
Anyway, this probably isn't something for the Avast forums to figure out. Sorry for wasting everyone's time.