40 mph is based on the speed from Wings of the Soaring Wind.
I have something a problem with Rego Corpus being able to achieve the exact same effect as Wings of the Soaring Wind (without all of the finesse, unnatural wind and requisite requirement issues), but that's more of a personal sensibility of mine than anything else. I peg "quickly" as 15 miles per hour (which for the paradigm is darn fast compared to walking or running). I've had this discussion here in the forum before. My sensibility is that while spells from different forms can mimic spells of other Forms, some things should just be done better with the right Form (tool). Corpus quickly had been undefined until TME, the authors, for whatever reason, decided that quickly, in the context of corpus, was the same as the speed granted by Wings of the Soaring Wind. So now a Rego Corpus spell makes a better flying spell than the definitive flying spell listed in the core book.
Rounds in Ars Magica are 6 seconds, therefore there are 10 of them in a minute (last paragraph of Combat Sequence section on page 172 of the core book).