Why is Dale Steyn not bowling fast?

Question

He was pegged as the fastest bowler in the world when he got into the Banglore Royal Challengers. Even Mallya said in an interview that Rahul suggested him because Dale is the world's fastest bowler nowadays. If he was actually bought because he is that fast, then why has he stopped bowling fast? Even RP Singh is regularly bowling faster than him. I wanted to see Dale bowl at his menacing best, but he hasn't made any mark in the IPL whatsoever. Why?