In real world, your web page consists more than one db (like mysql + redis + some RPC calls to microservices) queries, with async apis, you can concurrently request for all queries at once and join them all at rendering.
The async benefits can add up to a much faster responsive server.
a program with threads can support multiple requests simultaneously.
a program with green threads can support multiple requests simultaneously.
You arn't giving any reasons why green threads in python perform better than threads in the OS.