Queue Size specifies the number of outstanding (yet to be serviced) connections that the server can have. For heavily loaded systems (with many users) that have limited request processing threads, adjust this setting to a higher value.
Setting the connection queue size too high can degrade server performance. It was designed to prevent the server from becoming overloaded with connections it cannot handle. If the server is overloaded, increasing the connection queue size will increase the latency of request handling, and the connection queue will fill up again.