It would be very unlikely that its an issue with mIRC because people would have reported it by now, the likely events are either;
a) You are on several (or even just one), very large channel that has extremely large content, i.e. everyones talking at once, in which case this is how a "flood" would work, sending masses of data as a result freezing/slowing mIRC.
b) The other maybe that your using a script that is doing some sort of /whois or /who in/on the channel/users everytime you/someone joins and is slowing you down.
Eamonn.