I have not been able to reproduce this so far. The connection is immediate in my tests. Which version of Windows are you using?
UPnP is unlikely to be the cause of any issues since UPnP is only used when mIRC needs to listen for an incoming connection not when opening an outgoing connection. However you can try disabling UPnP in the Options/Connect/Options/Ports dialog to see if that helps.
Also in the Ports dialog, if you disable the Port Range options, does that help?