I got KB4589212 installed without my doing on both Win 10 20H2 and 1809 systems (the KB equivalent for the latter version). Result: Shadow of the Tomb Raider CPU performance degraded by ~4% on my 6700k, even though the initial Spectre and Meltdown mitigations are turned off via InSpectre: ~98fps vs. ~94fps (double checked, don't mind the slight differences in perspective or NPCs) So I've blocked KB4589212 via wushowhide.diagcab. I wonder if the MC update will be forced on users with upcoming Windows 10 versions. What to do then, except of sticking around with old Windows 10 versions? There is an article by Microsoft about sidechannel attack mitigations, but it's a wall of text and it might still not be fully transparent either. I hate Windows.