If I recall correctly, QWERTY was designed to minimize jamming. The myth is that it was designed to slow people down.

Whether it does slow people down, as a side effect, is not as well established since, as another person pointed out, typing speed isn't the bottleneck for most people. Learning the layout and figuring out what to write is. On top of that, most of the claims for faster layouts come from marketing materials. It doesn't mean they are wrong, but there is a vested interest.

If there was a demonstrably much faster input method for most users, I suspect it would have been adopted long ago.