Bug 251016

Test case: playing https://www.youtube.com/watch?v=mBw7vkcQOew in 1080p and above resolutions ("stats for nerds" reveals it is playing using the VP9 codec on my computer)

Here are the specs and parameters of my main testing computer (but it was observed on various other people's computers as well, particularly on HiDPI screens):

* HP Spectre X360 laptop (15" 2018 model) with 4K HiDPI display, featuring an 8th generation (Kabylake) Intel Core i7 CPU and the associated intel graphics chipset, 16 GB of RAM and NVMe SSD
* Running on the Wayland GNOME session on Fedora 37, with GNOME's night light & dark mode turned off (just to make sure this can't interfere)
* Testing with the "technology preview" Epiphany Devel flatpak package from the gnome-nightly repository for consistency/simplicity

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## Reproduction

1. Go to https://www.youtube.com/watch?v=mBw7vkcQOew (or any YouTube video page, really) with Epiphany 44 and use the top-right YouTube menu to turn on dark mode
2. Now that dark mode is turned on, click the cogwheel in the video player's toolbar to access the "Ambient Mode" toggle, and make sure it is turned on
3. Try to play the video for more than 10 seconds, or to interact with any part of the UI.

## Results

* Pretty slow on non-HiDPI (ex: 1920x1080) screens, noticeably more than if Ambient Mode is turned off
* Downright unusable when running on a HiDPI (ex: 4K) screen: sidebar thumbnails load very slowly, any interaction with the YouTube UI (even just trying to reopen the top-right menu to turn dark mode off) is nearly impossible as every single click takes somewhere like 20 to 60 seconds to happen (I didn't bother timing with a stopwatch, because it's painful enough to test this and try to escape the death grind), video playback cannot happen reliably, CPU is eaten alive and laptop fans are on full blast.

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Note: this is different from bug #248224 because what I'm observing here is specifically about Ambient Mode being turned on, and it's not just scrolling performance nor just playback nor just UI, it's all of them.