Bullshit. Qt is much faster than Electron, the Mumble client is really fast on my Turion laptop, that with OpenBSD.
And I say this even if I prefer Barnard IRL.
1) Would need to be lowest-common-denominator by nature
2) Would quickly stagnate due to friction against changes/additions
3) Would have few allowances for platform HIGs
If it were permissible to have vendor specific additions on top of a common core, that could probably work fine otherwise this hypothetical standard UI library would share many of the problems suffered by Qt, wxWidgets, etc.
The other option I could see working is something like SwiftUI, in which some control over the behavior, layout, and presentation is ceded to the platform — basically having developers provide a set of basic specifications rather than instructions for every pixel on-screen.
As for the free aspect, I feel like this ship has sailed like 20 years ago. Nobody will pay for an UI toolkit these days. This is not Unreal Engine 4, you know. That stuff only works on AAA games market, apparently (although I am curious as to why it doesn't work everywhere else -- likely thin profit margins and/or middle management greed outside of the gaming genre).
Crazy you say? Start making a list of the features a modern UI toolkit has to have to even be considered for serious projects.
Blame OS vendors for refusing to get together to specify a cross-platform standard API for UIs. We have mostly standard APIs for networking, file I/O, even 3D graphics, but not for putting a window on the screen and putting buttons on it.
OS vendors are still trying to play the lock-in game by forcing everyone to write GUI apps for only their platform. This is a non-starter, so everyone goes to Electron.
There are a few third party cross-platform UI libraries around. They suck. Qt is as bloated as HTML-based UIs, and then there's wxWidgets which is ugly and has an awful API based on 1990s MSC.
We could have something better, but it's an extremely large and difficult project and nobody will fund it. OS vendors won't because they don't want cross platform (even though all developers and users do). Nobody else will because nobody pays for dev tools or building blocks. The market has been educated to believe that stuff should all be free-as-in-beer.