Owin WebAPI Max Concurrent Connections

Question

public IHttpActionResult TestSpeed()
{
     Sleep(10000);
}

Called the above API concurrently, I can only manage to squeeze approximately 16 connections within 12 seconds. I think this is kinda low.

Is there any hard cap concurrent limit in Owin WebAPI Selfhost?

async is not an option for now, since it requires me to change hell lot of existing codes.