This is one of those slightly odd ones. I had high CPU on my PC even when not running anything, 20-25% (one whole core in essence) taken up by “System”. Mouse responsiveness was sluggish, sound would stutter, the works.
I tracked it down to ndis, and my Ethernet driver, using Sysinternals Process Explorer. Other folk have had similar experiences, so I updated the Ethernet driver. Things got worse. Rolled back and disabled all power savings functions on Ethernet. That helped a little, but only a little.
After a few weeks of off-and-on trying to fix this (and business travel in between), it occurred to me that my driver might be behaving just fine, that it was just genuinely busy – that there might be some form of traffic flood on the wire that caused my symptoms.
Sure enough. SSDP (UPnP discovery) packets to an obscene degree, originating from another machine in the network, with the string “Canon” in the discovery.
This was the Canon EOS Utility 2, or rather the UPnP discovery part of it. I saw no way of disabling UPnP discovery in settings, and the user was opposed to uninstalling the utility, so I just renamed the UPnP discovery exe so it couldn’t be started on boot, then rebooted the machine to check that worked. Problem solved. By killing the UPnP discovery process on another PC, my PC is responsive again.
That UPnP discovery process misbehaves. Until Canon fixes the issue – if they will – renaming it is the only workaround I can see.
If your System process CPU is high and it’s your network driver that’s doing it, run wireshark. It might not be your system at fault.