The QWERTY mechanical keyboard layout was devised in the 1870's. From all possible keyboard layouts it happened to become the universal standard (with a few very minor local variants in some countries, such as the AZERTY layout in France and some French speaking countries.)
One of the most persistent urban legends about the layout is that it was devised to slow down typists because the hammers of the mechanical keyboards were hitting each other and thus hindering typing, when typists were becoming proficient and typing too fast. This factoid is usually cited as evidence to why the QWERTY layout is (deliberately) inefficient and slow. (Unsurprisingly, it's a factoid often repeated by advocates of the Dvorak keyboard layout, who allege it to be significantly more efficient and faster to type with.)
The myth is based on truth, but only partially.
It is true that when testing different keyboard layouts, with some layouts the hammers, especially those physically close to each other, were hitting each other and getting stuck if typing too fast. After all, each hammer, controlled by its own key, has to hit the same spot, so if one hammer doesn't get out of the way before the next one comes in, it will be in the way and block it, or they could get stuck together, hindering or stopping further typing.
The QWERTY layout was indeed primarily designed to address this problem, by spreading out the most commonly used letters in English as far apart from each other as possible.
However, this was not done to slow down typing. It was done so that the equivalent hammers would likewise be as far apart from each other as possible. When two hammers in a mechanical keyboard are more apart from each other, they get out of the way quicker, giving way to the other hammer (since the hammers are physically located far apart in the mechanism, their trajectories diverge more quickly).
In other words, the QWERTY layout was designed to physically spread out the hammers for the most commonly used English letters, minimizing jams. It was not designed to slow down typing.
This is once again a factoid that's partially based on reality but gets the details wrong.
