Windows 10 19H1, an update already being tested out by some users, is now expected to contain new adaptations to the Spectre-patch problem. These fixes for the more severe Spectre variant 2 vulnerability can reduce performance by up to 30%. The solution is known as the Retpoline kernel.
https://www.notebookcheck.net/2019-Windows-update-may-address-Spectre-patch-related-performance-issues.339646.0.html