A possibly relevant argument is that Qwerty was not designed to slow people down, it was designed to stop the typewriter hammers hitting each other when typing quickly, which is achieved by trying to get the hands to alternate (hammers coming from opposite sides are much less likely to collide).
Since that seems to be a principal desideratum even for modern electronic typing, Qwerty got lucky and is possibly already close to optimal.
A possibly relevant argument is that Qwerty was not designed to slow people down, it was designed to stop the typewriter hammers hitting each other when typing quickly, which is achieved by trying to get the hands to alternate (hammers coming from opposite sides are much less likely to collide).
Since that seems to be a principal desideratum even for modern electronic typing, Qwerty got lucky and is possibly already close to optimal.