As is often the case with new versions of Windows, the idle CPU load and active CPU load of Windows 10 is much higher than Windows 8.1 With an idle Windows 8.1 VM (showing 0% CPU in task manager) the Activity Monitor CPU of that process in OS X is approximately 3%. A similarly idled Windows 10 VM uses at least 11% CPU in Activity Monitor. Both VMs are 2 vCPU 6GB on a quad core i7 MBP with 16GB. Had the same experience with Windows 8 when it was new, comparing Windows 7 to Windows 8.
Actually, this seems to be a Windows 10 issue. I loaded these VMs into VMware Fusion 7 trial and got similar if not worse idle CPU numbers for Windows 10.
