Best CPU TTS that can run on something like a raspberry pi is Piper. It can do real time synthesis on a raspberry pi and on a real computer it runs several times faster with negligible performance cost. I use it for 'reading' ebooks when my eyes get tired. The quality is roughly on par with where Mac OS's TTS was ~10 years ago (the last time I used it.) You can tell it's TTS, but it's good enough that you can become accustomed to it fairly easily.
https://github.com/rhasspy/piper