For sure it depends on the requirement/use cases, but I think most performance issues have nothing to do with the framework. It's usually N+1 queries, missing indexes or no caching etc... Fix those first.

People chase FastAPI for speed, but if your bottleneck is the database or any network bound work, using async vs sync framework barely matters. Premature optimization is real.

Flask handles production traffic perfectly fine. And by the time it doesn't, you will know exactly where the real bottleneck is.