I use fastmail, and it's pretty fast. I too have wondered what was under the hood, but I figured it out a while ago [1]. The docs are pretty old, and it could use better examples, but its a solid library. If you look at the code, it won't take long to figure out how everything works. They also open sourced JMAP[2] which is the synchronization protocol they use.
[1] https://blog.fastmail.com/2014/12/20/open-sourcing-overturej...
[2] http://jmap.io/