Similar concept and similar speed, but notmuch is a little more actively developed. I also find the notmuch command line interface a bit easier and the various tools built on top of it to be better. Alot, the terminal UI I mentioned in the parent comment, is almost exactly what I want in a mail program.
- http://www.djcbsoftware.nl/code/mu/
- https://www.emacswiki.org/emacs/mu4e