How can full-time lecturers find the time to prepare for lectures?

This question is mostly out of curiosity (at least for the moment). I have never taught full courses but I have had to substitute many times, usually for graduate level courses. Preparing for a one hour lecture took me 2-6 hours.

I understand that full-time lecturers may be expected to teach 20 hours per week. I cannot imagine how one could find the time to prepare. How do they manage?

Also, what about a freshly hired lecturer? Teaching a course for the first time should take considerably more preparation, creating an extreme workload with 16-20 hours teaching per week.