Aconcagua refers to the highest mountain in the Americas, located in the Andes. It's a very specific and unique proper noun to find in a pop song.
In the lyrics, Emilia sings, "A estas alturas siento que me falta el aire / Como en el Aconcagua" (At this point I feel like I'm out of breath / Like on Aconcagua). She uses this powerful image to convey an intense feeling of being overwhelmed and breathless, comparing her emotional state to the physical challenge of being at a high altitude on a massive mountain.
Emilia and Tiago PZK's "Rápido Lento" is all about that intense, secret connection between two people who just can't stay away from each other. It's a fiery, physical attraction that's been going on for a while, even if their hearts aren't quite as involved. They sing about how "siempre que nos vemos hay un punto de encuentro, una dirección, donde llevamos los cuerpos y no el corazón" [every time we see each other there's a meeting point, a direction, where we take our bodies and not our hearts].
It's a song about those moments when time seems to stop, no matter if things happen "rápido o lento" [fast or slow]. They're breaking rules, but definitely not the law of attraction! Tiago PZK even calls her body his "religión" [religion] and her sweat his "agua bendita" [holy water]. It's a super steamy track that explores that undeniable pull and the thrill of a secret rendezvous, where the physical connection is everything, and they want to keep it on the down-low: "Mantenlo callao', que mi corazón se duerme" [Keep it quiet, my heart is sleeping].