I am not sure I understand your bug report. Can you be more specific? Can you describe a step by step method to reproduce the issue? If you connect to an IRC network with two copies of mIRC and send messages between them, are the messages delayed?
You can use the "/debug @debug" command to open a debug window that will show you in real time which messages mIRC is sending and receiving.
Unless you are using a script, mIRC will send all messages immediately and will display all messages the moment they are received.
