What version of Redhat Enterprise Linux (RHEL) do you have?
An essential element for reasonable performance is an php cache, like PHP-APC
If memory serves me correctly, it is not possible to install PHP-APC and PHP 5.3.3 at the same time on some versions of RHEL.
You can have an earlier version of PHP and have PHP-APC, but that version is too old for Moodle.