The braced left leg is key to bowling your quickest. When it is braced you go over the leg with venom and the ball is fired at the batsman. It acts as a pivot that catapults the bowler through their action.
When the leg bends, it cushions the action and momentum and that pivot is lost. Its still possible to bowl quick but not as quick as possible
Its the one thing I hate about my action currently. I have a bend in my left leg that never used to be there. Number of surgeries, stress fractures and arthritis (sp?) in the ankle have meant it has been an unconscious (sp? sorry its too late 4:30am to check spelling
No doubt a bent front leg reduces speed.
A thing of beauty