This. Flask can technically run in an async event loop (see: http://docs.gunicorn.org/en/stable/settings.html http://docs.gunicorn.org/en/stable/settings.html)

This. Flask can technically run in an async event loop (see: http://docs.gunicorn.org/en/stable/settings.html http://docs.gunicorn.org/en/stable/settings.html). But gevent is a massive monkey-patch on top of Python standard libs to make them async friendly. If you're hitting enough load where async really matters, just skip Python altogether. And please, please, don't use Node. Just use Golang or Scala for true multithreading, and you'll save yourself the headaches Javascript will bring for something that's not truly parallelized.