be quick on the uptake

be quick (or slow) on the uptake

be quick (or slow) to understand something. informal
See also: on, quick, uptake

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.
See also: on, quick, slow, uptake