You can't just use Flask straight out of the box. You need to put it on something. I've used Gunicorn with Gevent and it runs as fast as I need it to run. Gevent with Nginx as a reverse proxy play very well together and the responsiveness is more about how you're managing your DB queries rather than rendering the site. (I shy away from ORMs for this reason but still use sqlalchemy.)
Both are great but I find them very slow.