be quick/slow on the uptake
be (a little) slow on the uptake
To struggle to understand something. This phrase is often used as a euphemism for stupidity. Joe's a nice kid, but tutoring him is a challenge because he's a little slow on the uptake, you know? That group is slow on the uptake, so I don't know if they'll ever understand how to use this fancy new printer.
be quick on the uptake
To be able to grasp or understand something easily. She's pretty quick on the uptake, so I'm sure she'll figure out the directions you left her.