Sometimes(though I don't know exactly what triggers it). DIsplaying an MOTD when connecting to a network causes mIRC to temporarily freeze, and then it soon unfreezes and is usable. I have skip MOTD on connect selected.
It's probably because mIRC can't reject the MOTD, server sends it but mIRC just do not display it. And if MOTD is very big, you have to wait Lag is also possible.
