The performance improvements of WebKitGTK aren't small, in my personal benchmarks on like MotionMark, performance has doubled in recent years and that is before Skia.
To be clear WebKit is made up of multiple ports. These ports are maintained by their own groups and Igalia maintains WebKitGTK/WPE where this change is happening. It does not affect the Apple ports of WebKit.
To answer your question though. WebKit is Apple's project and they do the majority of contributions. Igalia is the second largest contributor and collaborates with Apple regularly. Within the GTK/WPE ports Igalia controls them.
You are misreading the Phosh page. It says it uses a custom wlroots compositor on that page. You can see the source yourself: https://gitlab.gnome.org/World/Phosh/phoc
Yes it uses the GTK toolkit but that has nothing to do withe GNOME-Shell or its performance problems.
Plenty of consumer features could exist, like sharing actual high bitrate streams of your desktop or camera to others or syncing large amounts of data to servers for backup/sharing.
You miss the point. Those features do exist, and technically savvy folks like us use them (and they work fine with my asymetric connect). But the vast majority of users want YouTube/TikTok/etc to stream without a glitch and couldn't care less about backups, and as far as "high bitrate streams", pretty sure most users are thinking "works good on my iPhone, is good enough".
I wondered how this could reliably distinguish between a scene cut and a cut to commercial without content hashes and/or program schedules being shared through the network, then realized that 20 years ago was already 2003 and of course home internet was common by then.
Apparently one offline technique was checking for black frames inserted by local stations.
