Generally, it does improve the transaction per second of your API based on the official benchmark provided. Applications that benefit the most are those with lots of asynchronous tasks, IO-bound and high-level structured network code.

It is highly recommend to start adopting FastAPI for new projects or small systems. If you already have a stable system built on Flask and it does not contains much asynchronous tasks in your code, I would suggest not to rewrite it unless you have the time as you might need to maintain both servers during the transition period.

You should migrate an old system if and only if you are looking for standardization, data validation and auto documentation.