I agree! I've tried to explain it to people before myself, and it looks like he has the same struggle to get people to understand as I've had; it takes 10 mins of talking about it for people to come to grips with the idea, and even then they don't know if you'r right.
But I agree. Dogs don't differentiate between a faster action and a slower action. The end result is the same for them, they don't KNOW they did it faster cos they don't have a brain that compares their actions to their previous actions to find what is better. They get faster when they are in a high drive and somewhat expecting the command. End of story. No matter how amazingly well trained an action is, if you 'throw off' your dog and he's totally not expecting that command right then, then it'll take him a second to do it. He'll still do it fine, it will just take him a second longer cos he was totally thrown off by your command. It has nothing to do with if you only reward the fast actions, he'll still take a second if you totally throw him off, regardless of if you're trying to teach him to respond faster or not. Not-rewarding that slower reaction won't make him realise the speed is what wasn't right, cos the next time you ask he's in the mindset for it so will be ready and faster, NOT because you didn't reward the slower one, but because he's ready.