Microsoft warns of Outlook Classic bug that can crank CPU use up to 50% when typing

Microsoft has confirmed a bug in Outlook Classic, where users are experiencing CPU usage spikes, as high as 50%, when simply typing in the application. Notably, reports of this incident can be traced back to November last year, and even now, users have been asked to switch to the Microsoft 365 Apps update channel as a temporary workaround.

After around six months since the first sighting, Microsoft is finally addressing an issue in Outlook (Classic), which led to increased CPU usage, visible slowdowns, and even freezes whenever you'd sit down to compose an email or type a message. Microsoft quotes a figure ranging from a 30-50% hit to the CPU utilization, and that's sure to be noticeable, especially on aging hardware.

The developer team was able to reproduce this bug on updating to Microsoft 365 Apps Version 2406 Build 17726.20126+, which was released in June 2024, on the Current, Monthly Enterprise, and Insider Channels. As of writing, a concrete solution is not available, so users have been recommended to move to the Semi-Annual Channel release, where this issue has not been observed.

If you're running an organization with several devices, Microsoft offers a detailed guide for migrating your update channel with options like Group Policies, the Office Deployment Tool, Microsoft Configuration Manager, and Intune, just to name a few. For home users, a simpler way to achieve the same result is by just tweaking the registry as follows:

1. Open a Command Prompt (CMD) window with admin privileges.
2. Paste the following command and hit Enter: "reg add HKLM\Software\Policies\Microsoft\office\16.0\common\officeupdate /v updatebranch /t REG_SZ /d SemiAnnual"
3. In Outlook, navigate to File > Office Account > Update Options and select "Update Now" to switch to the Semi Annual Channel.

Affected users went through several troubleshooting hoops, like turning off graphics acceleration, disabling the spell-check utility, along with add-ins, but to no avail. It got so bad that a user with a beefy i9-14900HX reported CPU temperatures breaching 95 degrees Celsius just by having the 'New Message' window open, which is used to compose emails.

As the problem is still under investigation by the Outlook team, this is no more than a stopgap solution. The nature of this bug seems to be tied to how the software handles text fields, but it's quite puzzling how such an obvious issue slipped past the developers in the first place.