Slow on the Draw
Slow to understand or figure something out; slow to respond or react;
not alert; not a deep thinker.

In the American West of the 1800s you often had to be quick on the draw (get your pistol out of the holster fast) to stay out of trouble. If you drew your revolver slowly, you were a candidate for the undertaker. Eventually, the term “slow on the draw” was extended to include actions of the mind and not just the hand.

While his high school teachers thought he was slow on the draw, he graduated from college with top honors.
(From the “Dictionary of Idioms,” Marvin Terban, Scholastic Inc., publisher, 1996)