If you've recently updated or your PC has self updated to Windows 10 OS Build 18362.329 (run winver in a command prompt to find out your build) and Cortana has high CPU usage, download my . reg fix, extract and apply to resolve the problem (you'll need to reboot).
If you're one of those tin foil hat kind of people, then this is what is contained in the .reg file :
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Search]
"BingSearchEnabled"=dword:00000001
If you're one of those tin foil hat kind of people, then this is what is contained in the .reg file :
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Search]
"BingSearchEnabled"=dword:00000001