If it takes a long time for you to act or move, you're a slowpoke. Being a slowpoke is a real disadvantage when you're running in a 50 yard dash.
The informal slowpoke is a good one to use when your little brother is lagging behind, slowing everyone down: "Hurry up, slowpoke!" In the UK, the equivalent word is slowcoach, which makes sense if you think of a stagecoach moving slowly. The poke part of slowpoke comes from a rare meaning, "a device attached to pigs and sheep to keep them from escaping," and is also the root of pokey, or "slow."