The Task Manager tool is not reporting accurate CPU usage after upgrading to Windows 10 October 2018 Update. Microsoft acknowledged the bug in Task Manager last month when it released the RTM Build 17763 to the Windows Insiders in the Fast and Slow Ring. The Fix is expected to arrive on Tuesday with a fix for the bug and other issues. Windows 10 version 1809 is gradually rolling out but you can manually download and install the update without waiting anymore by checking for updates in Settings > Update & Security > Windows Update.
Source: https://www.bleepingcomputer.com/news/microsoft/task-manager-shows-incorrect-cpu-usage-in-windows-10-october-2018-update/