I think the 50mm f/1.8 argument here can be summarized in a variety of ways:
1. The focusing motor on the 50mm f/1.8 is slow, not even close to the speed of USM. Therefore focus is also slow, sometimes too slow.
2. Focusing accuracy varies. Accuracy depends on:
a. The camera body. Better bodies can focus more accurately than cheaper bodies. This is because better bodies are better at determining when something is actually in focus than other bodies. For example, the autofocus capabilities of the 5D are better than that of the XT.
b. The lens. Some lenses are not calibrated correctly for the specific body. In this case people may find front-focusing or back-focusing issues. The solution to this is to return the lens to the manufacturor for focus calibration. This is a pretty common problem.
c. Subject movement. If the subject is moving too rapidly, and the camera and lens autofocus speeds can't keep up, then you'll encounter a lot of miss-focuses.
d. Available light: If the light entering the lens is too low, the camera will have a hard time getting a focus lock.
e. Contrast on the point of focus: If there is very low contrast where you're trying to focus, the camera will also have a hard time getting a focus lock.
Miss-focuses are made worse if you're shooting at a large aperture like f/1.8. Because the depth of field is so shallow, correct focus is paramount. Therefore, it could be said that a bright/fast lens that constantly miss-focuses is essentially worthless.
Unless otherwise specified, 99% of photographers will define the term "fast," as in "a fast lens," to be equivalent to a lens that has a large aperture, and can let in a lot of light. This term is synonymous with "bright."
If one is talking about a lens that is slow-focusing, photographers do not call that lens "slow." Instead, one must specify that the focus is slow. ie. That lens' autofocus is slow or That lens is slow-focusing.
When talking about lenses, using just "fast" and "slow" in any other manner besides referring to aperture size and brightness can lead to confusion.