The delay does not come from a mirc option, because nickserv is not standard and can be set to any language and because mIRC does not offer a custom match string to compare against nickserv's message, mirc has a delay before joining channel to make sure you're auth before..
I'm not sure if mIRC has some correct understanding of the raw 402 and this is a bug, or if it simply does not handle it at all, for which there's the argument that it could, and the argument that khaled wants to avoid handling different custom raw per network.
Check this thread:
https://forums.mirc.com/ubbthreads.php/topics/265317/
