As a candidate, I've always preferred polite "thanks, but no thanks" responses in a timely manner. The other kind of feedback that I like is things like, "we need people with a different background / experience than you have." IMO, judgement feedback (you screwed up how you handle your yak shaving, or I don't think we'll get along with each other) is too subjective to be constructive. And, I get it. As someone who's…
Now, not every one became awesome. But, there are places for even the non-best developers to be productive. Good management is putting best employees on critical systems and others in supporting roles.