[eregon]

Interesting, I did not know that just calling the original method, even when the refinement method is not used occurs such a cost on CRuby. Seems worth reporting if not already done. No such thing on TruffleRuby though, the only peak performance cost would be for megamorphic calls (rare, even more so in combination with refinements).

[byroot]

> Seems worth reporting if not already done.

I assumed it is expected overhead.