But the 30WPM may "Squeek" between crasheds to result in a full copy.  Since there are "Slow" and "Fast" variables in conditions, and we usually are not aware of what the conditions are at the other end of transmission, I submit that the Speed selection is a Gamble.  The 20 WPM may be the best Guess.  Obviously 3 WPM and 40 WPM inherently create their own restrictions.
But I would agree that about 20 WPM would be my opinion as to this hypothetical "Dilema."
No scientifically based Pat answer, in my thinking.
But, Hey, I haven't operated in several years, what do I know?
73,  Nice to see some old friends as I played contest in the 160 and ARRL..  It's humbling to call someone with a 26Ga wire up 25' on Top Band......But Still Fun!
Val, N4RJ