Windows 10 ran into some issues as people started reporting massive CPU usage in some situations. All of this seems to be related to a problematic update, KB5412941, that made Cortana bring CPU usage to about 40 per cent even when idle.
Cortana is not exactly a popular Windows 10 feature for those who prefer their CPUs to perform in optimal conditions. This led to many Windows 10 users disabling Cortana through registry editor or .bat file usage. All of was fine and dandy until the KB5412941 update popped up.
This problematic update was supposed to fix a Visual Basic issue, which is quite ironic since the fix caused problems and now needs to be fixed itself. People noticed the horrible influence this update had on their PCs and eventually posted the problem and a workaround on Reddit .
People who previously disabled Cortana through the aforementioned means also reported the spike in CPU usage but the linked post on Reddit should provide people with a decent workaround. Just keep in mind that registry editing is risky if you don't know what you are doing, in which case we would suggest going for a different option, with lower chances of messing your OS up.
You can roll back the update by going to Settings, then Update and Security, followed by Windows Update and then clicking on View update history. There, you will have an option to click on Uninstall updates. The problematic update is KB4512941, so select it and click Uninstall.
Microsoft are aware of these issues as they wrote in the update release notes and are working on a fix. As Microsoft put it, the issue happens on devices that have previously disabled SearchUI.exe, or in other words, searching the web with Windows Desktop Search.
While the company didn't provide an exact date, they did say that they expect to come up with a solution by mid-September 2019. That is still more than a week away and if you are too annoyed with the higher CPU usage, you may want to try one or both of the aforementioned workarounds.