If you do see any performance benefits it likely will be because you support less features than what a modern day browser supports with HTML/CSS. But in my opinion you could probably achieve faster speeds with Canvas in a similar manner.

You have to be ambitious, right? :)

It depends on what you are doing. Rendering thousands of sprites with independent rotation and scale is easy. Rendering text is a lot of work, but once you do it you can do all sorts of things with the text. Rendering simple things like bezier curves is very difficult. There are some things in the 2D world that HTML/CSS/GDI/GDI+ are very bad at, compared to OpenGL, and vice versa.

All web content on OS X and iOS is rendered using OpenGL. That's not to say it was easy to make efficient, but it definitely outperforms the default CoreGraphics implementation on most content.

Though as an aside, I think voice control is very underutilized in modern GUI development...

I just wonder how well text performance will be. Would be awesome to get near native speed with editors like Brackets and Atom.