By default, Fn uses "cold functions" where every request starts up a new container, feeds it with the payload then sends the answer back to the caller. You can expect an average start time of 300ms per execution to start the function/container.
Hot functions improve performance by starting a function then keeping it alive to handle additional requests. This makes it a bit trickier to use because you'll have to parse a stream of requests, but if you use an FDK it's all taken care of for you, so you should probably use an FDK in most cases.
In your func.yaml, add format: json.
From there, we recommend using one of our FDKs to handle all the parsing and formatting. But if you'd like to learn about the nitty gritty details, check here.