JSON encoding test at TechEmpower, CodeIgniter vs PHP.

The most beautiful thing about CodeIgniter is its performance comparing to other PHP web frameworks.

TechEmpower published round-11 of web frameworks' performance benchmarks. Two of the benchmarks comparing Raw PHP and CodeIgniter showed big difference in the performance.
single-query (even if the raw queries were used without the Active-Record or any ORM) and json-serialization.
The reason for the slow performance for single-queries (as I read their code used for the tests) was the use of CodeIgniter-2 (but a bigger reason was) and using mysql driver in CodeIgniter and for PHP they used PDO driver (I reported that issue).

For the JSON serialization I read the method "set_content_type" inside the Output class, but I don't know why it is this much slow comparing to raw-php. Can anyone tell me why does this function run slower comparing to the raw-php code?