Actually, it's not supposed to slow you down. It's supposed to make sure you're normally hitting keys that are far apart, which means the hammers won't interfere with each other. QWERTY is a pretty good layout because of it. The next letter you try to type is rarely in the domain of the finger that just pressed down, meaning you can line up a number of keys to be pushed at once.

In the Design of Everyday Things, Donald Norman talks about keyboard design a lot. QWERTY is really good, so switching to layouts that are only slightly better (dvorak) end up not being worth the time and effort.