Basically your looking at the interaction of a number of factors.
First of all you have the camera body. The 5D range design was never intended to be used for "action" photography. From user reviews it strength is image quality especially in low light but a common gripe is about focus. I understand that the focus system is basically that used on the x0D range. So not exactly the bees knees in the Canon line up for ultra critically focusing applications. But for your application I suspect it will be good enough.
The auto focus motor speed. Canon developed a Ring Ultrasonic Motor (USM) which is virtually silent and extremely fast. They are fitted to their more expensive lens. A micromotor USM is fitted to their cheaper range which are noisier and somewhat slower. The penalty of going for a cheaper Sigma equivalent was to suffer a very slow auto focus motor. Some recent models of Sigma are now fitted with a fast HSM focus motor.
The distance the motor has to travel to cover the entire focus range of the lens. This is dependent on the lens focal length and its focus range. This can be a considerable distance on close focusing telephoto lens and very short for wide angle lens. Some lens are fitted with a focus range limit switch to restrict the lens working range so that the motor is not travelling from one end to the other all the time.
Contrast. Basically in non technical terms an auto focus sensor is looking for the point where the image contrast has peaked. It needs to see a difference in light levels from the subject before it can get to work. This is why it is almost impossible to focus on blank scene such a sheet of white paper. In general the more light on the scene and passed through the lens will give the sensor a better start position due to the improved contrast. With wide aperture lens a factor to be taken into account is its optical performance wide open. A common issue seen on some 1.4f and 2.8f lens is reduced contrast wide open and reported unreliable focusing issues in low light. Google can be your friend to track down and avoid such problem lens.
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