Speed wise it almost certainly doesn’t matter – any decent C++ compiler is just going to pass a single int.
The important point is readability – which will make your code more obvious to the reader?
If it’s obvious that these enums are really just ints then I would pass them by value, as if they were ints. Using the const ref might cause a programmer to think twice (never a good idea!)
However – if you are later going to replace them with a class then keeping the API the same and enforcing the const-ness might make sense.