The father of the typewriter was Christopher Latham Sholes, who ran headlong into an unforeseen problem. His early machines, slower than the typists' fingers, kept jamming. To correct this problem Sholes carefully devised the QWERTY keyboard. He spread the most common letters---E, T, O, A, N, I---all over the board and ensured that frequent combinations such as "ed" had to be struck by the same finger so that the machine would not jam. In other words, the QWERTY keyboard was invented to slow down typing speed. -- William Hoffer: "The Dvorak Keyboard: Is It Your Type?", Nation's Business, Vol.73, No.8 (August 1985), pp.38-40.
The most frequent combination in English is "th", which ordinary has to be struck by different fingers. The second is "er"+"re", and the third is "he"+"eh". "ed" is less than half for "th" (cf. Roy T. Griffith: "The Minimotion Typewriter Keyboard", Journal of the Franklin Institute, Vol.248, No.5 (November 1949), pp.399-436). "To slow down typing speed" is nothing but a hoax of Mr. Robert Parkinson, as I mentioned before.