Learn Spanish (Latin America): A ellas les importa llegar rápido; en cambio, nosotros preferimos caminar. - It matters to them to arrive quickly; however, we prefer to walk.

The phrase en cambio means 'on the other hand' or 'however.' While pero (but) simply links two ideas, en cambio is used specifically to strongly contrast two different attitudes or situations. Here, it highlights the direct opposite between their rush and our desire to walk.

Why include the pronoun nosotros if preferimos already means 'we prefer'?

You could leave it out, but using nosotros here makes the sentence sound much more natural. Because you are using en cambio to contrast two groups (them vs. us), adding the subject pronoun strongly emphasizes that contrast.

Why is it importa (singular) when the sentence starts with A ellas (plural)?

The verb importar works just like gustar. The actual subject of the verb is the action of arriving quickly (llegar rápido), which counts as a singular thing. The phrase A ellas les is just the indirect object telling us who it matters to.

I thought rápido was an adjective. Why isn't it rápidamente?

You can absolutely say rápidamente, but in everyday spoken Spanish, it is very common to just use the adjective rápido as an adverb to mean 'fast' or 'quickly.'