My first guess was some new security mitigation.
Like here:
But that should only affect amd.
EEVDF replacing CFS would be another good guess … but its supposed to be better not worse, of course.
Another big change with 6.6 was Intel Shadow Stack (CET,SHSTK). Maybe related?
This has an easy toggle; use boot option nousershstk
Also, attached are ~warnings~ that this is a security feature, blah blah, etc.
Heres the kernel documentation:
https://www.kernel.org/doc/html/next/x86/shstk.html