First before I go into a few things, I look forward to seeing this addon develop and I'd love to hear success people have had with UI.X themes. I'm happy to help support further development and wish the developer the best of luck!
However, I've read almost ten pages of UI.X bashing by this developer. And just to weigh in: yes the point of UI.X is to add features. It's a feature adding framework. If you want a lightening fast theme, disable those features in UI.X or use a theme that doesn't use those features. Removing these features in UI.X by toggling them off will get you back to the default style + material styling css.
I could argue, or bash, the nulumina themes or other theme designers (as theme designers in general seem to be a bit more dramatic than most) for their questionable tactics also. But things change over time. In fact, the AC.UI theme we are waiting to rebuild for the next major release for XenForo won't utilize features like JS based node grids, dozens of different one-click ways to move your header, breadcrumb, canvas styles, hundreds of style properties, etc. Anything that made life easier for admins that liked tinkering, if it's even remotely superfluous it's getting the axe!
These days, speed is far more important than giving admins the ability to customize every little thing and add all kinds of modern features the default theme didn't offer. For it to be knocked for that is simply a lack of understanding of what the goal was when it was built. It accomplished what it set out to do with flying colors.
That said I don't disagree that there is a time to focus on what is needed for the modern web in the present day. My opinion has changed strongly since I built UI.X almost ten years ago. I do plan to release UI.X 3 in the near future and will be focusing on performance and intuitive modern UX primarily. Add-ons like this one we will fully support!
Best of luck on this, thank you for supporting UI.X, and I'll be giving it a try myself!