I going to guess that there is no material difference between A and B. No need to even consider C, unless you are already using a script for other reasons.
If you have a modern, reasonably fast Mac, then I doubt you could detect the difference.
However, if it is that important to you, then I'd suggest you run some timing tests, in a real-world environment. Each Mac is different, and it could make a difference on a highly loaded, slow Mac.
If you could be more specific about "firing off lots of stuff", then we could probably be more helpful.
Underneath, the menu item is directly tied to an Action method, which will execute as soon as the menu item is selected. So I have no reason to think that would be slow.
But I think a shortcut key would have to resolve to a menu item, and then be executed.
Six one -- half a dozen the other.