Interestingly, fast glass means a more accurate AF lock, but not necessarily a faster one. An f/5.6-sensitive AF point will actually achieve a lock faster than an f/2.8-sensitive point. With an f/2.8 or faster lens and a dual-sensitivity AF point (either an f/2.8 line + f/5.6 line cross type, or a dual cross type), focusing is a 2-step process - an initial focus move from an f/5.6 sensor line, then a refinement by an f/2.8 line. In AI Servo mode, most of the work is done by the faster (but less accurate) f/5.6 sensor lines.