I have tested the test file you gave and can confirm it works as advertised.
It delays the import by the set amount of time, all downloads are completed successfully.
It appears as if UE looks at almost every file separately and applies the delay to that file while importing files it saw earlier and already had that delay applied to it.
It's very sleek and effective.
Though sometimes it imports a few files at once that all had the delay applied to it, like it collects them or something, but that's probably just internal organizing or something and still makes it work as planned.
Great work!