Buggy update KB3156421 slows down Windows 10 PCs/Laptops

Microsoft does it again with another update KB3156421 for Windows 10 that makes your PC/laptop “abnormally slow”

With the big Anniversary Update coming to Windows 10 in July, Windows Insiders have been receiving regular new builds packed with new features and improvements. KB3156421 is the latest cumulative update released by Microsoft for Windows 10 PCs and phones bringing a wide range of fixes and improvements, and bumping the build number up to 10586.318. Unfortunately, some people are reporting it’s causing serious slowdowns.

However, Microsoft itself has announced yesterday that the latest cumulative update comes with a known bug that makes several PCs very slow once it is installed on Windows 10. Fortunately, there’s a possible fix if your PC is affected.

Microsoft’s John Wink, Senior Program Manager Lead working on Windows 10 updates, said on Reddit that “if you find that your PC is abnormally slow after installing the update, you may be seeing a problem that I’ve been trying to track down for a couple of weeks. Here’s something you can try to see if it alleviates the symptom. I’d love to hear the results of this test, and could really use your help in gathering additional information if indeed it rectifies the symptom for you. Please give this a shot and post your results to this thread. Thanks!”

The simplest way to fix this slowdown issue caused by the update is to simply disable Cortana. In order to disable Cortana, click in the Cortana search box to bring up Cortana. Then open the Notebook and hit the Settings button. Move the slider under “Cortana can give you suggestions, ideas, reminders, alerts and more.” to “Off”.

However, if still want to continue using the personal assistant on your computer, there’s still no fix available for the moment.

In reply to another comment in the Reddit thread, Wink also noted:

It’s actually not a very wide-spread issue, it’s just that when you hit it, it’s incredibly annoying. If disabling Cortana resolves the symptom, that means we know what’s happening and we’ll probably roll a fix into a future update. But I need some examples of cases where this workaround works to justify rolling that fix in. 🙂

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Kavita Iyer
Kavita Iyerhttps://www.techworm.net
An individual, optimist, homemaker, foodie, a die hard cricket fan and most importantly one who believes in Being Human!!!


  1. Have HP with AMD 6120 and 10gig of ram system ran find with Window 8 barely would hit 10% cpu in task manager. Installed windows 10 home and at first it ran pretty good. Then I believe the anniversary update system slowed down. Before checking the boards I started looking around and started shutting tasks down in task manager. Didn’t make any difference. Turned of Cortana and performance was better but nothing like before windows 10 update. I noticed even though Cortana is turned off it still shows up in task manager. Also
    my system really heats up if I play any kind of video. Fans just kick in to max speed. Not sure what’s up yet, can’t find an app that will show me everything that’s active. I mean everything so I can track all i/o and processes. Any help is much appreciated. Thanks



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