Another factor is that by mentoring my own output goes down a lot. If I get interrupted several times a day to help then I get nothing done on my own stuff. I think on overall it's still a big gain for the team but I look worse.
That calls for setting clear boundaries, and proactively checking in/interrupting your mentee when it's a good time for you to take a break.
I like metrics based on how much time the other person has and expects to waste not being able to do anything productive, which strongly suggests you should give him at least two things he can be doing, an obvious second one being reading documentation or otherwise researching necessary stuff he doesn't yet know. After a certain number of hours it's worth an interruption.
And there's the reverse effect that teaching someone else forces you to better learn the subject, so you should gain some extra productivity from this.
