I think you are oversimplifying this. If you don't know how FastAPI handles sync code, then you can't claim it is on par with Flask. I suggested it might use an executor, after looking in their code, they use a thread pool support in Starlette, which in turns imports this thread pool logic from AnyIO. I see no easy way to determine what is the size of the pool. Clearly this isn't designed as a main way to run code, it is just a quick way to run sync code w/o blocking the loop. I would not consider this up to par with the concurrency you get from Gunicorn or uWSGI using a configurable thread or process pool.