>>The threat—even an imagined threat—of a large bill made it difficult for poor people to focus on the cognitive tasks at hand.

This issue happens because one has to constantly juggle many priorities in the mind all at one time. This can come down very heavily on a person. To have to make decisions to chose one over the other, a.k.a making sacrifices can be soul depleting if done for a very long time.

There are often conflicting priorities many demanding attention at the same time. In case of trade off you often stand to lose something for the other.