Actually, many police departments either ordered their revolvers or modified them so that they CAN'T be cocked single action. Generally, a cocked double action revolver in the hands of someone with a dose of adrenalin on board is a bad thing. Not only is an unintended discharge far more likely, the chances the gun handler will forget it is cocked and reholster it that way is pretty good. A double action revolver is best cocked only when the decision to shoot has been made.

Proper training with a single action semiauto is to remove the safety after the draw, while raising the gun up on target. It doesn't slow anything down, as it takes less time to flip the safety off than it does to raise the pistol- doing both at the same time slows nothing. Same thing when lowering the gun from a target, muzzle down/safety on. Up-snick, down-snick.

Proper training in good technique and frequent practice is always the key to safe use of a defensive handgun.