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Updated, 9/6/19, 8:45 a.m. PT: Microsoft acknowledged the CPU spikes caused by the KB4512941 update to Windows 10. The company said the problems listed here--abnormal CPU usage from the SearchUI.exe process and the absence of search results from the Taskbar--are only affecting "a small number of users." (Which doesn't mean a whole lot when there are more than 825 million active Windows 10 devices around the world.)
According to Microsoft, this problem "is only encountered on devices in which searching the web from Windows Desktop Search has been disabled," which has become increasingly difficult after the Windows 10 May 2019 Update. The company said it's "working on a resolution and estimate a solution will be available in mid-September." It's not clear if re-enabling web search via Windows Desktop Search can address the issue.
Original article, 9/1/19, 9:22am PT:
Here's an evergreen lede for you: a cumulative update to Windows 10 has caused performance issues for some users. Windows Latest reported Saturday that problems with Windows 10 Build 18362.329, which debuted with the KB4512941 cumulative update, have caused high CPU usage issues.
KB4512941 was released as an optional update on August 30. Microsoft said in a knowledge base article that the update was supposed to address numerous issues affecting Remote Desktop, Windows Sandbox and other aspects of Windows 10. (The company also said that it would be phasing out the Edge browser's support for ePUB files, which are typically used for e-books, "over the next several months." More information can be found here.)
Windows Latest said that numerous people have complained about high CPU usage problems after installing the update, however, purportedly because of an issue with Cortana. The virtual assistant's SearchUI.exe process has reportedly used 30-40% of the CPU as well as 150-200MB of memory since the installation of the KB4512941 update. But those resources are being totally wasted--the Cortana window opened from the Taskbar fails to load.
It's not clear why Cortana decided to be a processor hog with the KB4512941 update. What is clear, however, was the fact that members of the Windows Insider Program reported these problems to Microsoft via the Feedback Hub before the update was made available to the public. Those reports appear to have slipped through the cracks; now people who install the optional update are sacrificing a significant portion of their CPU to Cortana.
Issues like these have become something of a trend with Windows 10 cumulative updates. An update in April led to performance issues, a May update broke Windows Sandbox, and an August update caused networking problems on Surface devices. Other cumulative updates have simply failed to resolve the problems they were made to address. At this point it's harder to explain why people should install these updates than why they might want to wait.
Windows Latest said that some users resolved the problem with Cortana by deleting a registry key, but unless one of the update's improvements outweighs the performance hit, it's probably easier to simply uninstall the KB4512941 update until Microsoft addresses the issue.