We'd seen rumblings about the October security and feature update patch that Windows 11 automatically downloaded and installed for users. Among other reported issues, it for some reason disabled USB keyboard and mouse connections when entering the Windows Recovery Environment (surely you've kept some old PS/2 devices handy, right?). But that pales compared to the absolutely bizarre testing results we recently confirmed regarding PC gaming performance.

As it turns out, the October patch, identified by its KB5066835 patch number, left a number of users scratching their heads in regards to performance in Assassin's Creed Shadows. At the time, the wild frontier of Steam comments about uninstalling monthly Windows updates, complete with all of their built-in security updates, may not have seemed like a sound path to better PC performance.

It wasn't until Nvidia directly addressed KB5066835 with a recent GPU driver hotfix that we finally raised our eyebrows and took a look. Nvidia blames this October patch for 24H2 and 25H2 versions of Windows 11 in its notes to users, saying that it may lead to "lower performance" in "some games."

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One of a series of benchmarks we measured in Assassin's Creed Shadows with KB5065835 left untreated at first, then treated via a recent Nvidia hotfix patch.

Having seen complaints in the wild, we booted AC Shadows on one of our normally patched Windows 11 PCs and immediately saw remarkably low frame-rates on a Ryzen 7 9800X3D PC with an Nvidia RTX 5090 GPU. We recorded footage on the affected PC, installed this week's Nvidia hotfix, then recorded again in the same regions of the game, spaced out by using save points from our original review of its PC performance.

The KB5066835 Windows Update, left untreated, sliced AC Shadows frame-rates from 33 percent to over 50 percent.

Determining exactly which other games have been affected is not easy or obvious, mostly because Nvidia has opted not to broadcast its own findings for us to test and confirm. We haven't found much information about other games whose performance was directly affected by KB5066835, with an exception of Counter-Strike 2 suffering more dips and stutters in frame-rate before having the hotfix appiled.

But this scenario certainly has us scratching our heads as far as what kind of testing regimen and communication with GPU makers take place on at Microsoft these days. Did Microsoft's October update expose a huge flaw in GPU drivers that OEMs needed to fix? If so, why was Microsoft unable to steward that information more responsibly to companies like AMD and Nvidia with weeks of woeful performance existing in the wild?

Or, in the opposite scenario, why is Microsoft breaking performance levels in a manner that GPU manufacturers have to correct on Microsoft's behalf after the fact?

Either way, this is only the latest Windows gaming headache the Digital Foundry team has faced in a year full of them. As such, we're beginning to turn our minds around on the matter. Putting our heads together to come up with a non-Windows solution - for ourselves and for our PC gaming readers - isn't necessarily an easy thing in terms of drivers and overall game compatibility, and it may take time and effort to come up with something satisfying. But that effort still might be a little less crazy than repeatedly suffering the slings and arrows of outrageous Windows installation misfortune. Stay tuned.