be slow on the uptake - to be slow to understand new ideas.
Some of the games were quite complex but the children were very quick on the uptake.
be quick on the uptake
To be able to grasp or understand things easily. She's pretty quick on the uptake, so I'm sure she'll figure out the directions you left her.
be quick (or slow) on the uptake
be quick (or slow) to understand something. informal
be ˌquick/ˌslow on the ˈuptake
(informal) understand things quickly/understand even simple things with difficulty: He’s a very good worker but he’s a bit slow on the uptake sometimes. You have to explain everything twice.