This issue has been discussed in previous posts. UPnP can be slow and possibly unreliable depending on your UPnP setup on your system and/or your router. You can google UPnP to learn about similar issues with other applications.
If you disable UPnP in mIRC and it reports that UPnP is not supported, this means that UPnP is not working correctly on your system. I would recommand downloading UPnP testing software (see my previous post) to monitor UPnP activity. This will tell you whether UPnP is correctly setup on your system and router and it will also show you exactly when mIRC adds/removes UPnP ports.
