I did not see any images that I thought would tax the autofocus of any Canon or even a third party lens.
If a photographer is capturing a series of photos of a subject rapidly moving toward him and needs them all to be in focus so he can catch a critical moment, thats where excellent and fast focus makes a difference. Then, its a combination of body and lens that makes a difference. For example, 1 series bodies use more voltage to drive the focus motor faster than say a 5D MK III can. They have a 12volt battery versus 7.2 which is a huge difference.
Using one shot AF, try focusing on something close, then point the camera at a distant object and press the shutter fully. The camera will take the image when the lens is focused. Compare that with a 85mm f/1.8. You will see a huge difference in the time it takes to focus.
However, its only that one initial focus that takes the long time. Subsequent shots that are more or less at the same distance, focus will be fast. Thats why its not a problem in actual use.