I'm going to guess that the northbridge might have something to do with it and it isn't just a CPU issue. I say that because I've tried to help a friend troubleshoot it. They used the integrated sound at first which was southbridge. Southbridge has to go through northbridge to talk to CPU.

Had friend try to run audio off her graphics card only. Graphics card on her old motherboard has no direct CPU access, it goes through northbridge first. Still had issues.

Friend tried a 70$ soundblaster audio card. Said card still has to go through northbridge to talk to CPU and should have offloaded some work off the CPU. Still had the issue.

Maybe the changes to audio are jamming up the northbridge on older CPUs, as well as just not being good for those CPU designs.