> But the other database tests were run against the same DB server and configuration. How can Flask's performance be the DB's fault?

They are written against the core database layer or the framework (in case of Django the Django ORM) and in case of Flask: Flask-SQLAlchemy. The default config of both of these things is to not pool connections. That is not true for all other frameworks.

Also I'm not blaming anything here, just pointing out that there are differences in the setup.