Flash content plays using the H.264 codec. This will clock some graphics chips at 400 MHz for the Unified Video Decoder (or UVD for short), assuming you have hardware acceleration enabled. For example, I am able to force my clock speeds to stay at 735 MHz, regardless of whether or not the chip detects 2D, low-power 3D, or high-power 3D. Flash hardware acceleration can and will knock those down to 400 MHz.
When I'm not forcing clocks, they'll be at 157 MHz core when idled/2D, and 735 MHz when any 3D mode is detected. 400 MHz doesn't come into play anywhere with either of my power settings. Yet, 400 MHz is what my graphics cards will clock themselves to when a hardware-accelerated Flash video is open. I can't control this. No one can.
For example, with hardware acceleration enabled, it looks like this:
400 MHz. Once again, I can't control that with any settings. Even forcing 735 MHz through profiles doesn't work; it'll downclock to 400 MHz to play the video back using my GPU to accelerate things. 400 MHz is higher than my 2D clocks, though, which is why a very small portion of the player base may see a performance increase with this. Without hardware acceleration, my GPU's got pretty much nothing to do, defaulting to its lowest clock speeds:
Hardware acceleration can be great, and it can also be quite evil. The changed clock speeds can give you better performance or worse performance, depending on where sat before (i.e. if they were sub-400 MHz in WoW), but this is extremely rare and Flash normally cuts FPS by quite a bit. I get just a bit more than half the clocks when I enable it and play YouTube videos, basically forcing me to disable the acceleration to maintain high FPS in-game.
Note that if you're an ATI/AMD owner that seems to see better FPS with Flash hardware-accelerated videos, you can force your clock speeds to max out as well. More info here:
Another workaround for ATI owners experiencing oddly low clock speeds is enabling the D3D11 API. In DirectX 11 mode, graphics chips tend to stay at their max clocks.
tl;dr: Getting low FPS in WoW with a YouTube video open? Disable Flash hardware acceleration.
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