[aminsadeghi]

Pracically, most of the time, IMO, where learning speed matters, is when you're competing against someting, could be an individual, or a startup, etc, which doesn't seem to be the optimal condition one would like to be in.

[Jarwain]

First counterexample that comes to mind, you're traveling to a country for one reason or another, and it'd be helpful to learn or know more of the language. The faster you learn, the more helpful it'll be.

Another example, your friend's band's guitarist got sick and you've got a week to learn a full set's worth of music

Generally I find some urgency to be a nice motivator