Help with Site Performance

[PWillard]

Hello there,
I am trying to come up with a solution to my forum's page load performance as Google ranks it as 88% slower than other sites.
here's a screenshot showing Google Performance stats and showing the average server load
http://img809.imageshack.us/img809/7203/hostj.png
the forum is running on Centos v5.7 with vbulletin v4.1.8 with over 4 million posts and the database is using InnoDB barracuda format with MySQL v5.5.19 and PHP v5.3.8
The daily unique visits is around 2,500 to 3,000.

Installed Addons:

Cyb - Advanced Forum Rules
Cyb - Advanced Forum Statistics (admin display only)
Cyb - Auto Reply
Cyb - Sub-Forum Manager
Usergroup Color Bar
vBSEO
vBSEO Sitemap Generator
Advanced Permissions Based on Post Count v5.0
DP Sphinx



please help me fix the average 8 to 10 second page load time and to lower the server load.

[Disasterpiece]

You should install a php debugger like xdebug and run a script profiling. Then you should see which is causing the execution delays.