The reason the network degradation happened is that Microsoft couldn't figure out how to stop heavy network activity causing audio glitches on some systems even after giving audio realtime priority, so they hacked around it by adding a fixed 10,000 packets-per-second cap on network activity regardless of system speed or CPU usage (less if you had multiple network adapters). See
https://blogs.technet.microsoft.com/markrussinovich/2007/08/... This was just as much of an issue on multicore systems because the cap was unaffected by the system speed and chosen based on a slow single-core machine.
