Typing speed increases if there’s no feedback from the input (looking at keyboard) and output (looking at the screen).¶
I’ve been taking book notes of recently read books. While taking notes I realised that I type fastest when I look at neither the screen nor the keyboard.
When looking at just the book, I let my muscle memory (training) take over and get the fastest typing speed. There are a few errors—typos—but the writing speed makes up for them.
Looking at the screen is the next fastest mode of typing. It is probably slower when I’m copying text from the book since I have to constantly switch between the two, specially because I have to locate the cursor in the book every time. It is also slower because any typos are apparent immediately and create a dissonance hurdle in the brain, slowing it down.
Looking at the keyboard while typing is the slowest. The brain skips a lot of the muscle memory, or tries to reconfirm it, and tries to look for keys before typing. It may cause the least mistakes but is really, really slow.
Seems like a few other sharp minds have been thinking on the same lines. Here’s a post in the Financial Times discussing research on similar lines, but addressing impact of disruption by Brexit. Here’s their list of languages, though I recommend you read the full post: