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.
The thread I linked shows everything published about fupdate. If you type: /fupdate it shows the current value. Fupdate defaluts to 0 each time you start mirc, so any desired value should be in ON START or perform-on-connect. 100 is the maximum acceleration it does updating screen. In some cases you'll see the text blink and it's done, instead of a slow scroll. 0-99 are percentages of whatever 100 does, with 0 percent being 'off', the prior behavior.
It is unlikely that /fupdate will make a difference here. The issue appears to be the speed of the switchbar display, especially when large numbers of windows are open. I have looked into the switchbar display routines and cannot see a way of speeding them up. That said, I might be able to find a way of implementing a cache of some kind, which might help. I will be looking into this for the next version.
What are the sort of behaviors or activities performed by the switchbar that tend to consume CPU? I'm not encountering this, and I do quite a bit of hanky panky with hiding/showing switchbar icons based on channel activity or lack there of.
If a script uses commands that cause the switchbar to be updated often, that could be the cause of the issue. If you test your scripts with the latest beta, available from the bug reports section, do you still see the same issue? The latest beta includes switchbar optimizations that speed it up in a number of situations.