I've never seen anyone pick it up that quickly before

Cobb wants to do a job. The job is to plant an idea in someone else's mind. For this they first design the layout of dreams by an architect. They imployed a new architect (Ariadne). But first she need to learn some things. To teach her they take her to a shared dream to explain every thing for her. In this shared dream Cobb is the subject. dreams is populated by the subject's subconscious. When they (Cobb and Ariadne) dreaming, Mall (Cobb's dead wife, that is in his subconscious) attacks Ariadne with a knife in his hand. Ariadne became killed then waked up. When she wake up she says:

Ariadne to Arthur (Cobb's friend): I don't know if you can't see what's going on, or if you just don't want to...
but Cobb has some serious problems that he's tried to bury down there. And I'm not about to just open my mind to someone like that.

Then she goes and leaves there.

Cobb says to Arthur: She'll be back. I've never seen anyone pick it up that quickly before.

Inception movie.

Can you please explain the bolded sentence?