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I believe Canon some years ago had an article about the 300/2.8 IS being the fastest focusing lens (The new generation lenses probably changed that).
I have tried to photograph Swifts head-on with the 400/5.6, but they can easily outrun it when they get close enough. (The closer they get the faster the USM have to move to keep tracking, there is a reciprocal-value involved).
I tried the 300/2.8 IS, it is probably a bit faster, but more difficult to swing around, so I prefer the 400/5.6