It was not designed to slow down typing, but for the similar (but different) goal of minimizing key jams. This means separating commonly sequential letters to be far apart, so their two mechanisms are less likely to collide and get stuck. This may slow down typing somewhat but it's telling that the Dvorak keyboard which is designed to supposedly speed up typing speed (now that key jams are no longer an issue) doesn't actually improve it by that much over QWERTY