Yesterday, I've updated my systems to kernel 6.6.9 – two Intel-based desktops and one AMD-based notebook. When rebooting the latter, I immediately noticed that something was wrong. Logging in, for example, seemed to take twice as long, and the desktop needed much longer than the usual two or three seconds to come up. My Intel desktops, in contrast, behaved exactly as before.
To substantiate my feeling that my notebook's performance had degraded significantly since the update, I used sysbench, or, more precisely, the command
sysbench cpu run. I would normally see a performance of about 4800 events per second on one core. But with kernel 6.6.9, all I've got were 440 events per second, more than a factor of 10 lower than the Ryzen 5800H in my notebook is supposed to deliver, and even three times lower than my 10-years old Intel desktops. No surprise the notebook felt so sluggish!
I didn't bother to investigate this issue further, and I don't know the underlying cause, like whether it's related to the AMD processor or the maker of the notebook. I just rolled back to kernel 6.6.8 (
sudo pacman -U /var/cache/pacman/pkg/linux-6.6.8.arch1-1-x86_64.pkg.tar.zst) and the problem was gone.
I expected problems with kernel 6.6.6, but the devil is in the details.
Update: The performance is back to normal with kernel 6.6.10.