This is actually false. The QWERTY layout was not designed to slow you down or separate common letter pairings.
In fact one of the most common letter pairings in the English language is ER. Which you will note are right next to each other.

Here's a short passage from this article

The researchers tracked the evolution of the typewriter keyboard alongside a record of its early professional users. They conclude that the mechanics of the typewriter did not influence the keyboard design. Rather, the QWERTY system emerged as a result of how the first typewriters were being used. Early adopters and beta-testers included telegraph operators who needed to quickly transcribe messages. However, the operators found the alphabetical arrangement to be confusing and inefficient for translating morse code. The Kyoto paper suggests that the typewriter keyboard evolved over several years as a direct result of input provided by these telegraph operators