high traffic

I recently converted a somewhat big site to Drupal 6.

We currently have:
~13,000 registered users, usually anywhere from 50 - 100 online at a time.
~23,000 nodes
~380,000 comments

I'm running on a VPS with 4gb ram, 200 gb disk space.

I have drupal page compression enabled, optimize css and js enabled.

My host has installed xcache for me.

I really love the drupal framework but my users are very displeased at my current speeds.

Some of the complaints I have received are:

"Speed- slooooooooooow. Agonizingly slooooooooow. 20+ seconds per click."

Hi

Today I talked to an Sysadmin in my favorite coffee bar and he gives me tipp. They are trying out in the firm a new PHP Server called HIPHOP.
http://developers.facebook.com/news.php?story=358&blog=1

You seet it in the URL. Facebook has developed it and delivers 90 % of his webtraffic over Hiphop.

Here is my initial post about performance tuning our site http://www.divx.com/ This is mainly a place for me to brain dump and spark discussion and hopefully open up issues to track and get some of these fixes into Drupal core.

Servers

4 - Web servers with 8GB Memory each, Apache 2.2.x + PHP 5.2.x
2 - Memcache Servers with 8 GB Memory each. 9 bins per server
2 - Database Servers with 16GB each running MySQL 5.0.x

Traffic