I have 2 50mm lenses, the nifty 50 1.8, and the Sigma 50mm 1.4. Neither of these seem to focus very fast or very well in low light. At least my 70-200 f2.8 II blows them away, and my 24-105mm f4 IS is maybe halfway between the 50s and the 70-200.
Now the 50mm does give more light in the photo and lets you use faster shutter speeds, but it usually will not focus. I am talking about a shopping center parking lot (bunch of guys getting their cars together) and things like that. Yes I could use a flash, but that is not what I am after.
Logically it seems like the f1.4 or 1.8 should be a big advantage when trying to find contrast and focus, but it does not seem to work that way. Is the Canon 50mm f1.4 better?
This is with my 5D Mark III and 1D4. I used to have a 5D2 and 7D, and it seemed the same.