Yes, that’s it. I got answer for bug report:

"This is a feature: we let the user see the message and wait for 1 sec
after showing it, to make sure this particular message is not
immediately replaced by some others.  From after-find-file:

      (when (and warn msg)
        (message “%s” msg)
        (or not-serious (sit-for 1 t))))
                        ^^^^^^^^^^^^^
(The value of not-serious is nil in this case.)

This is not a bug."