And no, speed is not the most important. As long as the user can see the payment incoming, it's trivial UX to say "Payment received. Will be confirmed and available for use in 3 minutes."
edit: it appears MobileCoin is (allegedly) built on a combination of XMR + the Stellar consensus protocol? If true that's a slightly better scenario than I previously thought
Don't get me wrong, we stand on the shoulders of giants, but there's a lot of new tech here.