PHP 5.4 compliant

Hi I upgraded my site to PHP 5.4 witch is supposed to make a website load 30 to 40% faster! my rackspace reseller did the upgrade and it crash my website, we switched the site back to PHP 5.3 and we are trying to work out which plugin caused it.
Your help well be much appreciated

http://wordpress.org/extend/plugins/wordpress-seo/