slow on the draw
1. Lit. slow in drawing a gun. (Cowboy and gangster talk.) Bill got shot because he's so slow on the draw. The gunslinger said, "I have to be fast. If I'm slow on the draw, I'm dead."
2. and slow on the uptake Fig. slow to figure something out; slow-thinking. Sally didn't get the joke because she's sort of slow on the draw. Bill—who's slow on the uptake—didn't get the joke until it was explained to him.