Django and FastAPI are both Python web frameworks, and teams treat the choice as a fight. It is not one. Django is a batteries-included framework for building a whole product; FastAPI is a lean, async, typed framework for APIs and services. The honest question is which one fits the thing you are actually building.
Almost every production AI system you have used runs on Python. Not because Python is fast — it is not — but because the ecosystem, the model SDKs, and the data tooling all live there. The interesting question is what separates a Python notebook that works from a Python service you can run in production.
Django is not slow, and Python is not slow. Slow Django apps are slow because of the same seven decisions on every project — N+1 queries, missing select_related and prefetch_related, missing indexes, no caching, synchronous external calls, and an ORM used without watching the SQL it writes.