I've been having issues with mIRC for quite some time now and it's very similar or the same as yours.
The client keeps freezing, quite frequently, sort of like every 10th second or so, the freeze is about 2 seconds.
Not only that, the CPU load is abnormally high, in fact, mIRC is the process that uses my processor the most and it goes everywhere between 3% up to around 50% (!).
I unloaded a few scripts here and there but it didn't help, so right now I've just enabled simple scripts that shouldn't make the client freeze or use a lot of CPU. What we all seem to have in common are SSL connections and a lot of active servers?
I'm on 13 networks and two of these uses SSL (esper.net and a private one).
What to do -_-.