I have tested with a simple server and client setup using a single session in the same machine(tcp socket on local host). The client sent in new orders and got execution reports back. If the orders were sent at a high data rate for a period of 100-200ms, the round trip time for a new order and the corresponding report was not good.

My understanding is that there are two different threads in sending and receiving messages. What could be the reason causing QuickFix engine to slow down processing the incoming messages? Since there is no network overhead, it's most likely to be caused by the decoder and the way FIX message is constructed.