Be quick/slow on the uptake
Definition 1 of 1
Pro Tip 1/3Figurative Meaning
The phrase is figurative and doesn't involve physical speed but mental understanding. 
Despite the complex topic, Mia was quick on the uptake.
Pro Tip 2/3Not Insulting
Saying someone is slow on the uptake is not necessarily insulting but may suggest they need more time. 
Give him some time; he's a bit slow on the uptake.
Pro Tip 3/3Neutral Tone
This phrase is mostly neutral and can be used in informal and some formal contexts. 
Our new intern is quick on the uptake; she'll fit in well.