George Schlossnagle - Why PHP Scales - A Cranky, Snarky Answer:

"In every application I've ever worked on application performance was governed by the following factors, in rough order of their importance:

• stupid use of network resources
• stupid use of databases
• stupid algorithm design
• stupidly ignoring contention issues
• stupid language inefficiencies

Notice how PHP being naturally slower than Java (something I believe to be true) comes in last? That's a distant last."