Illustrator super laggy after Windows 11 upgrade

Have you tried updating your Illustrator to the latest version 29.6.1? 

I'm kind of dreading upgrading on the OS on my notebook computer from Win 10 Pro to Win 11, but will have to do so soon. Experiences like what you're having with your system underscore the stress of the situation.

If you haven't already tried resetting Illustrator's Preferences file try doing that as a first trouble-shooting step. Re-installing Illustrator after resetting the Preferences file would be a next step. If that doesn't work I'd try using the Creative Cloud Cleaner tool to completely remove Illustrator so it can be re-installed in a completely fresh state.

Also, make sure drivers for your computer's graphics accelerator board and other periphreals are up to date. Drivers can also differ from their Win10 and Win11 versions.

The other thing I worry about when upgrading operating systems is the OS upgrade itself won't work properly. If the OS upgrade turns into a performance down-grade then it might become necessary as a last resort to wipe the boot hard disc and install the OS clean (and re-install all the applications). Obviously that can be a big pain in the neck. Sometimes situations like that can turn into opportunities to do hardware upgrades, like install a bigger/better boot SSD and maybe even a second matching SSD if the computer has a drive bay for it.