I've found it useful in the past to keep a close eye on build times because it can be easy to introduce code over time that drives the build time up. By the time you notice that this is becoming a pain point, it's very difficult to track down the offending pieces of code to resolve the issue.
I think code documentation works best when it lives side by side with the code it is documenting rather than being written in a separate document. When it's separate, it becomes easy for documentation to become stale. Out of sight, out of mind!
Have you struggled to figure out a good way to test your Netlify Functions? As you start to make more use of functions, you really need to have good unit tests in place for peace of mind during production deploys.