How to switch user quickly, on Mojave?
On Mojave it is simpler than I thought.

Clicking the power/fingerprint button will quickly bring the login screen out, with the other user already chosen, ready for the password to be inputed.

After the login it will be possible to switch with just fingerprint-click + fingerprint (for authentication).

That's pretty convenient!

Note: I am not sure if that was active also before, or if the alternative user is chosen based on what are the most active/logged users (because, in my case, the proposed user for login is not the only other user present on the system).