<blockquote>If I type a common sequence like "er" or "th," I do it with a single flick of the hand, not four separate ones.</blockquote> Skilled touch typists certainly don't make four separate motions to type "er" or "th". Keyboards are specifically designed to accept multiple keys being pressed at the same time, because a skilled typist naturally presses the next key before they have finished the motion for the previous one. Nearly all keyboards will accept two simultaneous keypresses, with higher-quality ones accepting 3, 4, or arbitrary numbers of simultaneous keystrokes.

To be specific, typing "er" involves lifting my hand upwards, hitting "e" and "r" with my middle and pointer fingers in quick succession, and then dropping my hand back down. Typing "th" involves lifting my left hand at the same time as I shift my right hand slightly leftwards, and striking the "t" slightly before striking the "h" (though I often transpose the two actions and end up typing "hte" or "htat").