Best way to optimize for authenticated user performance? The keyword here is authenticated!

Currently (as far as I know) the de-facto ways to optimize for anonymous access are through static page caches (varnish/squid/boost).

However, as far as I know this doesn't seem to do much good in a scenario where most of your users are authenticated.

Besides the obvious APC, Memcache and throwing money at hardware, what is the best way to performance tune a high-traffic site with mostly authenticated users?

For example: drupal.org, social networks, communities, etc.

I'm more interested in Drupal 6 at the moment so I guess MongoDB doens't apply for now...
Every site is different, thus it will require different tricks to get it working. Here are the ones I use for our sites for logged in performance.


http://drupal.org/project/dbtuner
http://drupal.org/project/advagg  
http://drupal.org/project/labjs  
http://drupal.org/project/css_emimage
http://drupal.org/project/imageinfo_cache
http://drupal.org/project/cdn


If you REALLY want to know what is causing the slow code, get a cache grind of your site. What all contributes to drupal page execution time?

edit: Looks like the 7.x version of Auth Cache is starting to be developed. Haven't used it but something to look into.


http://drupal.org/project/authcache