Not sure what you mean by "ignoring async". If you write your handlers as regular functions, I would imagine FastAPI is going to send them to run in an executor. So all the concurrency benefits of FastAPI go down the drain, and now your concurrency is based on threads.

As far as Swagger integration, you can use APIFairy with Flask and get similar type of auto-generated docs. Disclaimer, APIFairy is an extension that I created: https://github.com/miguelgrinberg/APIFairy