That depends on the web site!
Nobody has done an analysis yet, but my gut tells me that the clearfoundation.com bottleneck is the database. A good chunk of the web site (forums, community stuff, some content) runs off of Joomla. Due the nature of this beast, there's a lot of database activity on a page load. Doing a quick check on our test server, I can see that loading the front page of the forums takes 483 (not a typo) database queries. Yikes! Very good hard disk performance and MySQL tuning is the name of the game.
The documentation section, on the other hand, uses
Dokuwiki. You could easily build a simple web site using just dokuwiki. No databases involved and it's very lightweight.