I dont have too much to add to the fingers coming off the bar thing, I think others have addressed this. In a nutshell, it seems to be a flexibility thing, which you can and should work to improve, you can also just work on keeping a good grip on the bar, John Thrush even advocates keeping the hook grip as you flip the elbows through to combat this...
As far as speed under the bar, in an ideal world, with no mental barriers, the bar doesnt have to be moving all that fast to get under it. Caleb has proved this to me several times... ive watched him complete lifts where it looked like he was in doubt of even deadlifting the bar, then boom hes under it. So I suppose I agree with Angel to some degree. Unfortunately, most will not go under a bar in these conditions, whether its possible or not.
Also, it does seem to have a lot to do with bar path/bar position... in "analysing" some of Calebs lifts compared to lifts by other lifters I have coached lifting similar weights who seemed to need a lot more bar speed, one difference is bar position and "balance" in the second pull position. so I do agree that proper placement of the bar in the second pull position has a lot to do with the amount of speed needed to complete the lift.