windowslatest[.]com/2019/08/31/windows-10-kb4512941-high-cpu-usage/
The 2nd cumulative update for Windows 10 1903 was released last week, but you'll only see it if you click to Check for updates. It includes quite a few fixes, including the Sandbox issue, but some users are reporting a problem with 30-40% extra CPU usage. The unofficial theory is that it's caused by SearchUI.exe [Cortana]. From the article:
Some users have claimed that deleting a Registry key, which prevents the Start menu search from sending local search queries to Bing, resolves the CPU usage problem.Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Search
Reg Name: BingSearchEnabled
Reg Value: 0
Yet Again this is something that was reported by Insiders, but in such low volume nobody noticed.