Microsoft has released the Windows 10 KB4515384 cumulative update that fixes a high CPU usage issue in Cortana’s SearchUI.exe process. The issue is only encountered on devices in which searching the web from Windows Desktop Search has been disabled. Users can just install the update and fix the issue while still keeping web search disabled. The update may cause other unanticipated issues, so be ready for anything, so don’t be afraid to install it right now. It’s also possible to overwriting Cortana folders with ones from older versions.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-fixes-windows-10-high-cpu-usage-in-cortana/