I managed to narrow this down to one specific channel causing the stall. No matter how many I join it only stalls when joining this one channel. The only thing out of the ordinary
that Sparkle and I found is that the topic contains "๑۩۞۩๑" (see image).
It seems mIRC is taking a long time to decode this the very first time you join the channel, but it doesn't when you /hop on the channel.
That character is not in the font I have set so it seems related to this item in versions.txt
59.Added support for UTF-8 display font substitution/linking, option
enabled by default in IRC/Messages dialog, should allow mIRC to
display all characters regardless of the font you are using,
assuming you have the required fonts on your system.
myndzi thinks it's probably a windows problem, but even if it is, isnt there a way mIRC could remain responsive while it's finding a suitable font for the characters it can't display?