Hi,
Service Host: Local System is a bundle of system processes, or call it a generic service hosting container. It includes processes including Windows Auto Update and many required system services would be running in it.
Open Task Manager. Expand the Service Host: Local System to see what items are using more CPU clearly. Then right click the high cpu process to open this service. Try to disable it to check.
Meanwhile, please perform a clean boot and disable security software temporarily.
How to perform a clean boot in Windows
https://support.microsoft.com/en-us/help/929135/how-to-perform-a-clean-boot-in-windows
After that open system settings, using “check for update” option to search if some security updates need to be installed.
Last one, you could use Windows Performance Toolkit(WPT) to capture the trace and analyze this trace.
windows performance recorder and analyzer will be included in WPT. Installing them to find which file may related to high cpu.
Hope this helps and please help to accept as Answer if the response is useful.
Best Regards,
Carl