[lectrick]

Have a test suite that is 95% unit tests. And structure it in a way that tests can run in parallel, across all cores.

FROM THE BEGINNING. Because it's nearly impossible to make this change after the fact.

Also, use a code profiler and commit the profile data to the repo! This will allow you to plot performance issues over time and watch for big upticks.

This alone will make your code orders of magnitude times better (and likely, faster). You will catch concurrency issues before they become a huge problem, your test suite runtime won't be as much as an impediment to your development process, and you will simply write better, more modular, more maintainable code by having to unit-test every component in isolation.