I've never heard of anybody complaining about Django
per se having performance issues. When people say Django is "heavy weight," I tend to think of how it bundles a lot of stuff for you, which can make it easy to build an app; while, at the same time, if you end up deciding you want to step slightly off the garden path and do things
juussssst a little differently than Django wants you to, you're probably gonna have a bad time.
OTOH, Flask is too small to impose much of a conceptual framework on you, and SQLAlchemy generally feels like a fairly thin layer of Python syntactic sugar over top of SQL. When I write queries in SQLAlchemy, it seems like they always tend to come out looking more like what I would write in plain SQL than queries in Django do. Granted, both ORMs can start doing things like joining tables behind your back, which can certainly cause performance problems, but that's a problem that's common to both, and not just Django. ¯\_(ツ)_/¯
