The Intel Core i9 13900K "Raptor Lake" Performance From Linux 5.15 To Linux 6.1

What's up with Linux 5.18 and newer regressing performance for a number of workloads?

It's on the CPU security mitigation front, sadly... Linux 5.18 and later is the point in this kernel testing where the EIBRS Post-barrier Return Stack Buffer (PBRSB) mitigation is picked up and appears to be the cause in those workloads why Linux 5.18+ is slower than 5.15~5.17 results. The Core i9 13900K tested relies upon the software mitigations for PBRSB-eIBRS. I hadn't previously looked much at the PBRSB-eIBRS mitigation performance impact due to my ever-busy schedule with limited resources.