I am still unable to reproduce this issue. Every time I have tested this out, mIRC has waited until it has received a reply from nickserv before sending the join command. The only reason this might happen is if the server delays the nickserv reply for over 60 seconds. This is how long mIRC will wait for a reply before deciding that something has gone wrong with the server and continuing with the perform/join process. Or if you have used /autojoin yourself to override the join delay. We would need to see a debug.log in your situation to determine what is happening.
