https://forums.mirc.com/ubbthreads.php/topics/262140/mIRC_Slowdowns_for_Massive_Cha#Post262140

per above post, does your usage go down when you use: /fupdate 100

does your cpu usage go down when you temporarily disable your scripts and aliases with: /remote off
and goes back up when you enable them: /remote on

When talking about cpu usage, it helps to know how many cores your computer has. Unless things changed in newer Windows, if you have quad-core, task manager showing 25% means it's using 100% of the core it's running on.