Autofocus needs light to focus and the more the better. A fast lens helps autofocus in low light for any given body as for every stop that the lens is faster twice the light enters the body and hits the detector. Lenses focus wide open then close down to the taking aperture after focus. The focusing motor for Canon systems is in the lens itself so better lenses will focus quicker. The 70-200 L zoom is a top lens so no doubt focuses quickly.
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