What it means
A slowpoke is someone who's forever moving at half-speed while the rest of the pack is already out the door. You throw it at the mate still tying their shoes, the kid drifting at the back, the person who turns getting ready into a whole side quest. It's usually playful, a little nudge with an eye-roll, not a proper insult.
Usage examples
"Come on, slowpoke, the film starts in ten minutes and you're still doing your hair."
"He's a slowpoke in the mornings but somehow always makes the train."
"Oi, slowpoke, we've missed the green light twice waiting for you to cross."
"Hurry up, you slowpoke, everyone's in the car and you're still looking for one sock."
Where it comes from
Slowpoke is recorded in English from the 19th century as a teasing label for a person who moves slowly or dawdles. It's built from slow plus poke, with poke used in older English for a dull, poky, or sluggish person. The word stuck because it sounds light, comic, and gently naggy.