SpeechT5 is a built-in text-to-speech (TTS) service that runs in the browser (Well, on Amica at least). It is free and relatively fast.
Make sure SpeechT5 is enabled for TTS:
Settings ->Text-to-Speech ->TTSBackend ->SpeechT5
Using SpeechT5
You can add new voices to SpeechT5 by adding new models to ./public/speecht5_speaker_embeddings/.private and running npm run generate:paths. You can download additional xvectors for SpeechT5 from here.