slow parse with rubinius 2.2.10 ?

Here's an odd one that I'd love some feedback on:

Attempting to squeeze extra performance out of an app, I moved it from MRI 2.1 to Rubinius 2.2.10. With a little tweaking, got all the tests to pass, but the ruby-protocol-buffers 'parse' seems to be about half as fast as with MRI 2.1. Does that seem right? 

I'm literally just taking a proto object and doing

```
object.parse(encoded_data)
```
