When I am debugging drupal, I came across the concept
page_cache_fastpath. I want to know the use of it.
I believe this was part of a technique for loading certain cached content from the database (thus requiring a db connection) without all the overhead of creating sessions, loading modules, etc.
Much of Drupal's functionality is unavailable while loading from fastcache which creates a tradeoff... load from cache more quickly but without Drupal being able to do things like run hooks or check permissions, or retain Drupal's core functionality while loading from cache more slowly.
You may also want to look at the boost module if you are looking for faster caching techniques