Videos on Wikipedia are not played smoothly when F12 is open

By design Issue #8486783


Xiaoyin L.
Aug 14, 2016
This issue is public.
Found in
  • Microsoft Edge
  • Internet Explorer
Found in build #
Reported by 2 people

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Steps to reproduce

To reproduce:

  1. Visit (or any other video on in Edge 14.
  2. Play the video. You can see that the video is playing very smoothly (assuming your network speed is fast).
  3. While the video is playing, right click on the video, and then click "Inspect element". When the F12 Tools window is open, you can see that the playing becomes slow and choppy. Then close F12 Tools, it plays smoothly again.
  4. Repeat steps 1-3 in Firefox and Chrome. In these two browsers, the video plays smoothly when F12 is open.

(1) This is reproducible only when playing videos on Wikipedia websites. Not reproducible on YouTube videos.
(2) Reproducible on both Edge 14.14393.51 x64 and Edge 14.14901 x64.


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    Comments and activity

    • The cause of this seems to be that video decoding is done using an asm.js module. Since asm.js is disabled in Edge when F12 is open, the decoding becomes slow. Although it would sometimes be handy for this not to be the case, this is apparently by design and probably won’t be fixed.

      In Chrome/FF, the page instead displays a native video element (which doesn’t use the asm.js module) as a result of feature detection since those browsers support native VP8/Vorbis playback while Edge currently does not.

    • Thanks for your reply, Ryan! But I don’t think it is caused by asm.js being disabled, because the issue is exactly the same in IE 11 on Windows 10 (slow when F12 is open; fast when F12 is closed), and IE 11 doesn’t support asm.js at all.

    • Microsoft Edge Team

      Changed Assigned To to “Ibrahim O.”

      Changed Assigned To to “Venkat K.”

      Changed Status to “By design”

    • Thank you for your feedback.  Wikipedia uses a JavaScript software video decoder to play WebM format in Microsoft Edge.  Microsoft Edge does not support the WebM format natively.  Opening F12 prevents the Chakra JavaScript engine from producing optimized code.  This enables web developers to debug the JavaScript.  The unoptimized JavaScript code leads to a subpar frame rate.

      All the best,
      The MS Edge Team

    • I see. Thank you for your explanation!

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