It's a 5% difference with only 10 keys. That seems significant to me.  (I wasn't expecting a much bigger difference, honestly).

Actually, I just ran that on 1.8.7, and with 10 keys it's a 15% difference.

Every little bit counts, and while I wouldn't sacrifice readability or maintainability for a a 5% perf boost of something that's not going to be a hot spot in the code...using a local in this method doesn't sacrifice anything, IMO.