It depends. Like everything else photography related.
Faster lenses (larger apertures) let more light to the AF sensor. The speed at that point, especially in Canon, is the speed of the motor in the lens - USM or not - and the gearing of the motor to the focus mechanism (Macros are geared for precision, not speed!).
In good light, it's generally going to be a moot point. In BAD light, a faster lens will mean better AF.
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