I like this. It makes a lot of sense. And I think you're right on with the mobile point as well. I've run into that problem in the past especially when passwords have characters that aren't easy to find with mobile keyboard interfaces.

What irritates me (as a user) more than delays, FWIW, are the sites that assume your username is correct and default to telling you the password is wrong vs. noting that it could be either one. I've wasted much more time hammering on passwords when the username was incorrect than I have from typos in the passwords. (Not that the delays aren't a nuisance.)