The build times are okay, I suppose. I don't really have anything to compare to. I believe they cache things like npm modules to speed up the build. Often the bottleneck is deploying to AWS itself (my staging environment is on a micro instance). My Lambda project builds are 4x faster... but they're going from TypeScript directly to ES5 whereas my Beanstalk project builds are TypeScript to ES6 to ES5 via Babel and the code base is much bigger (plus many more dependencies).
The build machines are supplied by CircleCI. It's all in the cloud.
I'm interested in getting into Amazon's Code Pipeline and Code Deploy stuff... but too many other things on my plate right now. Plus, I find that Amazon's platform and documentation isn't very approachable.