Speed of learning is much harder to quantify because people aren't uniform in the experience they already bring to the table. So you'd have to administer tests of some sort across a wide variety of things to encompass things they do and don't already know.
But the talk of "objective metrics" for the "quality of developers" sounds a little bit leading, making this whole question seem a bit off-putting. You might be well-served by reading "Zen and the Art of Motorcycle Maintenance", especially the parts where Pirsig describes the difficulty of assessing Quality without already being an expert in the subject.
So many times I have spent longs hours working on a very simple task but those long hours went in trying to find out the best, most elegant, most efficient, yet all inclusive (for cases), and still requires the minimum amount of code.
And once you get there, you get the feeling of eureka. You know that tini-tiny little piece of code can take anything you throw at it.
How in the world would you quantify that :).
Of course that's subjective... And impossible to compare dev a using tech 1 vs dev b using tech2.
I'd say as long as the person doesn't make the same mistake over and over and (maybe) doesn't ask the same question over and over, the person is fine.
Some people "crank it out", but... don't consider the bigger picture. (Don't understand and/or don't take time to consider context.)
Even when their solutions are technically correct, or "adequate", for the current definition and state of things, you may find significant problems down the road that someone else, perhaps not such a "speed demon", might have foreseen and avoided/mitigated.