Short version: there is no way to disable this, or to shorten the 1 second timeout :-( The good news is that you can just start typing while this message is being displayed, the characters are still inserted.
Longer version: from
if (msg_silent == 0 && !silent_mode)
ui_delay(1000L, TRUE); /* give the user time to think about it */
msg_silent is set when using
:silent, for example
:s/./a/ will give the warning, but
:silent s/./a/ won't. There is, as far as I can see, no way to start insert mode silently (
:silent startinsert doesn't work).
silent_mode is set with
-s Silent mode. Only when started as "Ex" or when the "-e"
option was given before the "-s" option.
But as the help message says, this only works when using Vim as
ex, and not when using it as