speeding up PHP

Hi there, I do have a performance question regarding php processing. There are some sites on the net like google, heise and others which are incredible fast in response time. Of course companies like google are running totally other systems, but there are smaller sites and companies doing almost the same verry fast page delivery. Now I am wondering how this is possible on a LAMP application. My site is not slow, but also not fast. I was reading about php accellerator. Can anybody recommend this tool, is it complicated to install and configure? Are there any speed gains and are ther tools to messure the page delivery times? Thank you for any information on that. regards, merlin