Mongrel was effectively so much better than using FastCGI and it served well for a time. Now passenger is a step in the right direction, easier to setup and monitor.

We had some memory consumption problems also when we decided to migrate and just switching to passenger did little for those. It was really the switch to REE that made the difference. Also the memory was not really from complex database operation, it was for rendering huge erb templates for the Googlebots. In the end there were no reason to always generate those templates and caching solved the problem definitively, but with REE it was already acceptable and we could have let it be for some months too.