use it to your advantage.
My ds, 3.6, loves pretending he's got a remote control like they have. So if he's dawdling along and we're in a hurry, I press my own remote control at him and say "FAST FORWARD!" and he instantly speeds up. Likewise if he needs to calm down, we press the remote and say "PAUSE!"
Sounds really corny but it does work, and is far more effective than nagging or shouting!