[tom_mellior]

We don't know that it's really I/O, as in "pushing some bytes to the system". All we know is that the author saw a hot method called "write" and stopped the analysis there. It might well be something like messing around with character encodings to get those bytes in the first place.

[tom_mellior]

Thinking about this more, we know that the author saw a hot "write" method in the profile for the fast run and doesn't have a profile for the slow run. The slow versions could be spending most of their time in a completely different place.