A recent update to Windows Server has caused Azure AD Connect to hog CPU time on some servers.
To fix this download the latest version of Azure AD Connect and upgrade your installation.
The upgrade requires no reboots or down time the only issue is that the sync process will be stopped until the upgrade is complete.
Upgrading the software typically takes 10-20 minutes and you will be required to re-enter your Office 365 global administrator password.
Once complete you shouldn’t have any issues with high CPU usage.