Disclaimer: I have not yet tried this browser, so I am not sure if it will meet your performance needs. I think it is worth looking at though, since it is fully open source and community driven.
Falcon
GPLv3
Looks awesome but the fact that updates are 1 year apart is a bit of a turn off lol.
Perhaps all that is needed to pick up the momentum is a spark of interest.
As an outsider I see how you can think that way, but alas Falkon is not a 'new' browser.
It's the ol' faithful Qupzilla, but rebranded to fall into the KDE umbrella. But as other KDE browsers (konqueror, rekonq...) it's dying a slow death only to fall in darkness.
URL: https://nyxt.atlas.engineer/
source: https://github.com/atlas-engineer/nyxt
license: BSD-3 & Creative Commons
Also, if you are an emacs fan:
https://ag91.github.io/blog/2021/06/08/emacs-nyxt-and-engine...
From their FAQ:
https://nyxt.atlas.engineer/faq
Nyxt is web engine agnostic. We utilize a minimal API to interface to any web engine. This makes us flexible and resilient to changes in the web landscape. Currently, we support WebKit and WebEngine (Blink).
So in theory, you could switch between web engines. I don't know if you can do that on demand or not. It does seem like a cool project, so I will likely try it out.