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 2/3
Not 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 3/3
Neutral 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.