If we assume that application is not trying to perform lots of IO operations concurrently, then IO dispatcher would not even have to create additional threads. Actually, it is a concern of an application developer to explicitly limit concurrency in application and to make sure that fast-users clicking users on a slow networks would not create lots of concurrent jobs inside the app. The limit on the number of IO threads is not designed for that. It just a limit (since some limit needs to be put).