What is the main difference between Flask and FastAPI in terms of asynchronous support?

The main difference is that Flask is synchronous and based on WSGI, whereas FastAPI uses ASGI and supports asynchronous request handling.

Flask is a synchronous framework built on the WSGI specification, meaning it handles requests in a traditional blocking manner. FastAPI, in contrast, is built on ASGI, which supports asynchronous web applications. The book notes that a separate project called Quart replicates Flask while adding ASGI support. Because FastAPI is ASGI-based, it can run faster than Flask in many cases, particularly for applications that benefit from async concurrency.