PUBLIC MARKS with tags sql & cache

For a fairly large community site (think OG, node access control, private messages etc), we were getting in trouble giving authenticated users a decent browsing experience. The worst page was taking 16 (yes, sixteen) seconds to load and others were getting in trouble too now that the number of users and nodes was growing rapidly. Not acceptable of course, so we went searching for solutions:

In this article I will try to give a view of what is the custom caching with php, why and how we can use it.