Toddler Two Speed

My 3-year-old son seems to have two speeds - too fast or too slow. Why can't we be on the same page? When we need to get out the door to go someplace, he moves like molasses. When we are in a crowded store, playground or parking lot, he moves like the Tasmanian devil. What gives? How do we switch gears? I try to allow enough time so I'm not barking orders like a drill sergeant but somehow he invents new stall tactics - poopy diaper, grabbing his sister's shoes and running away, finding a lost toy to play with. Forget trying to leave his day camp. He becomes the ultimate immovable force. He MUST play one last time with the class guinea pig, finish an art project or clean up toys. Not that he is keen on cleaning up toys at home. I spend my days saying "slow down" or "hurry up". I feel the same way about his growing up, too. Some days it feels like a blur and I want to keep him this sweet, cuddly age forever. Other days when I've changed 100 poopy diapers, calmed 99 temper tantrums and cleaned up 98 messes, I wish life had a fast-forward button. Talk about mixed messages!