In order track down what is causing the freezes, you would need to install a clean copy of mIRC in an empty folder with no scripts and default settings. The best way to do this is to install mIRC using the portable option in the installer in an mIRC folder in your Documents folder.
Once you have done that, try connecting to a few servers and channels and see if you can track down exactly when the issue starts happening.
mIRC performs a large number of tasks in the background. The main background timer for mIRC, which is triggered once a second, alone processes over thirty different features, from blinking icons to transfer timeouts and much more. In addition, every server event can trigger a large number of other features, some of which can be processor intensive, such as the Nick Colors and Highlight features. If you add scripts and SSL connections to the mix, that increases the processing time even more.
If you are connected to a large number of servers and have joined a large number of channels, all of these events and features combined will result in a high CPU load.