Learn Spanish (Latin America): Tienes que resumir la narrativa, lo que tomará tiempo. - You have to summarize the narrative, which will take time.

In this sentence, the word "which" doesn't refer back to a specific noun like la narrativa. Instead, it refers to the entire preceding idea of having to summarize the narrative. When you need a relative pronoun to refer back to a whole clause or abstract situation, you must use a neuter pronoun like lo que.

Could I use lo cual instead of lo que?

Yes, lo cual works perfectly here and means the exact same thing. Because there is a comma separating the clauses, both lo que and lo cual are grammatically correct for referring back to the whole previous idea. Lo cual just sounds a bit more formal or written, while lo que is more typical in everyday conversation.

Is resumir a false friend? It looks like "to resume."

Yes, it is a very common false friend! Resumir means "to summarize." If you want to say "to resume" (as in to continue an activity after a pause), you should use a verb like reanudar or retomar.

I've usually seen llevar tiempo used for "to take time." Can I use that instead of tomará tiempo?