Windows 10 Control Flow Guard may slow down your browser

Vivaldi recently discovered some major performance issues caused by a Windows 10 security option called Control Flow Guard. Microsoft is working to resolve the issue.

A hotfix is expected to be available with the Windows 10 May 2019 Update.

Right now, we can not say if this performance issue also affected the new Chromium-based browsers. 

What is Windows 10 CFG?

Control Flow Guard is basically a Windows Defender feature that is available in Windows 8.1 and newer versions.

A team from Vivaldi noticed that Chromium unit tests performed better on Windows 7 compared to Windows 10.

One of the tests that took 100 minutes when run on Windows 10 on this machine, took 20 minutes on Windows 7. 

Vivaldi informed Google about the issue and the company ran some internal tests. Apparently, a Control Flow Guard in the platform is responsible for the issue. The tech giant also confirmed that the company faced some issue regarding CFG in the past as well. 

After investigating the matter, Google further shared the result with Microsoft. The Redmond giant plans to release a fix in the next few weeks.

Bruce Dawson, a Chromium engineer at Google published his findings in a recent blog post. He stated the issue affects only large .exe files and is unlikely to affect Chrome. He suggests the issue can be resolved by disabling CFG. 

CreateProcess had O(n^2) performance for CFG data. Now it doesn't.
Timeline of this Windows performance bug:
April 15: Initial private report
April 21: Isolated repro and blog post
April 23: Fix built (flighting in a few weeks)https://t.co/PLsWMqeier

— Bruce Dawson (@BruceDawson0xB) April 24, 2019

The Vivaldi engineer Yngve Petterson, who identified the CFG flaw fears the bug may affect Vivaldi and Chrome as well. Dawson says Microsoft still has to work on some CFG issues. 

Never turn off Control Flow Guard

Most of you may prefer to disable the Control Flow Guard to gain performance improvements. However, this is not a recommended solution and it is better that you should stay away from it.

Windows 10 comes with many security features as compared to the previous versions of Windows. Exploit Protection is one such feature that protects your system from different vulnerabilities.

However, the bug was noticed with large processes that were launched multiples times in a controlled environment. You should not notice performance issues under normal conditions. 

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