Rendering the final triangles isn't the issue. The issue is that while your 3D world is nicely setup polygons that benefit from all the culling you can do with it, your 2D UI is requiring vector graphics which your GPU will absolutely hate having to render. The actual composition of the UI elements is incredibly non-performant. Pushing the pixels doesn't matter.

There's a reason why browsers are so complicated and equally non performant. 2D rendering, even GPU accelerated, with the constraints we have right now (or rather, the lack of constraints, because 3D rendering is basically tradeoffs all the time), is not going to be more performant.