There is a lot happening, sure, but none of it is logging.
Wrapping code with if (_log.isDebugEnabled()) seems like a great way to control whether a log message is generated, but even this adds overhead. In single instances, the overhead is negligible, but if you have a server doing this check frequently while trying to serve thousands of concurrent requests, the overhead is measurable and significant.
For that reason, there is little if any logging decorating the Liferay code, typically you'll only find it at key points where errors can originate outside of Liferay's control and due to customization or configuration.
So the TL;DR version, the best way to understand what is going on under the hood is by a deep code dive or live debugging.
