For feature implementation speed, function point analysis (http://alvinalexander.com/FunctionPoints/) is a good start. There are tables available that give rough industry-wide figures for hours per function point. However keep in mind these will change quite a bit depending on the size of the project, the competence of the team as a whole, unique constraints on the particular software, and the tools in use; we're talking a range of 1/2hr to 20hrs per function point, so you need to factor those items in. Also you have to be willing to break down the requirements in ways that are a little odd sometimes when dealing with more interactive applications (say, games versus spreadsheets) or that have long-running processes (daemons versus batch).
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.
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.