https://wccftech.com/windows-7-windows-8-1-high-cpu-usage/

Microsoft has issued new security fixes and general improvements to Windows 7 and Windows 8.1 following the release of cumulative updates for several Windows 10 versions. Among other security fixes, Microsoft has also addressed the issue that caused high CPU usage after the Spectre Variant 2 fixes were installed.

Windows 8.1 has received KB4467697 and KB4467703 (security-only update) bringing the following changes and fixes: