My phone, a several-year-old Moto G, a middling phone even when new, does a decent job at text to speech without noticable thermal or power issues. And I don't

My phone, a several-year-old Moto G, a middling phone even when new, does a decent job at text to speech without noticable thermal or power issues. And I don't have mobile data so it can't be that it's offloading it to a server.