Magento Commerce

I’m working on improving this magento site… and I’ve got results showing a delay in rendering where the CPU is at 100% http://www.webpagetest.org/result/120822…1/details/ can anyone explain what causes this, and if/how I can eradicate it? Cheers, Steve ( debian 64bit, nginx 1.2.3, php 5.3.16 using APC and memcache, percona mysql 5.5 )