This is most likely related to UPnP which can be enabled or disabled in the mIRC Options/Connect/Options/Ports dialog. You can find a discussion on this issue here
UPnP is extremely useful because it ensures that your IP address is set correctly for file transfers. It also opens the required ports on your router to allow you to send or receive files. Unfortunately, some routers can be slow to respond to UPnP requests, which may cause mIRC to pause for a second or two.
You can disable it in mIRC, however it generally makes life far easier, especially if you often have issues with your IP address being incorrect or you do not know how to configure your router to open the required ports for DCC transfers or chats.