There could be a number of reasons for this: one of the issues that has been reported by some users is that mIRC can use 50% cpu cycles when you place your mouse over a channel switchbar button. If that happens, you can move your mouse off the switchbar button to resolve it.

Other potential causes: you may be using a script that is running for too long, in which case you would need to unload your scripts one at a time to see which script is the cause of the problem. Also quite common is if you are using anti-virus software and are downloading large files; the anti-virus software will scan the file at the end of the download, freezing the application that downloaded it.