I'm not sure what's exactly the issue, What is your pc's specification and power supply wattage?
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I do not know if it is normal or not. This question exists for very long time.
But if you use DXVA, video card(or Maybe AMD cards?) will down to UVD clock which is 200~700Mhz or so depends on resolution of the video. I see the same behavior on my RX460.
Also,I do not know what behavior will nVIDIA cards do.
If you wish to not affect the game, disable v-sync and do not limit fps , let game runs on top fps that it can reach.(But maybe not always works.)
Or try some 3rd-party clock-lock tools, to lock the clock at top value.
Vsync doesnt help, when video is in the background clock is at 300-500mhz when playing a game, as soon as I turn off the video clock goes back to normal.
Disabled the Hardware Acceleration in browser and now it works properly. Its definitely a driver issue.
Here is another thread about this :
If you dig it more on internet:
This thread is from 2012:
I am starting to think if it is how it works on AMD's cards.