As an aside and spinners in general;
Why don't spinners develop a fast ball as a surprise delivery?
Obviously it will at best be medium pace but slowing the ball down is a thing so why not speeding it up. It looked like Joe Root did one as a bit of a bouncer in this series which had their batsman jumping who then stayed back in his crease for the next few overs. Although I don't remember him doing one previously albeit obviously that doesn't mean he hasn't. If you get known for it then a batsman may be discouraged to come down the pitch more often.
Why don't spinners develop a fast ball as a surprise delivery?
Obviously it will at best be medium pace but slowing the ball down is a thing so why not speeding it up. It looked like Joe Root did one as a bit of a bouncer in this series which had their batsman jumping who then stayed back in his crease for the next few overs. Although I don't remember him doing one previously albeit obviously that doesn't mean he hasn't. If you get known for it then a batsman may be discouraged to come down the pitch more often.