I think that talking too quickly is not necessarily the same as talking too fast. Speaking too quickly could mean that one hasn't given sufficient thought to what one wants to say which is different than talking too fast. Does the same difference exist in the italian?
alfaman1: I think 'rapidly' should be accepted and you should report it, since 'speaking fast' and 'speaking rapidly' are fairly synonymous to me at least. "Quickly" could also have the connotation of 'speaking out of turn,' or as I said earlier in another post, without sufficient forethought.