rspec/rspec-core — pull request #1008 by JonRowe (2013-07-21 10:59:01+00:00)
Title: Remove performance degradation caused by `DelegateClass`

I think `DelegateClass` is the cause of the performance degradation you're seeing @myronmarston, give this a try. I tried doing this without directly specifying the methods but that had a similar performance failing.