I've said this before, but I'm tooted to say it again.

Browsers were originally designed to layout and render documents. To handle all the various writing systems, cultural legacy, etc it takes extra effort upfront to do so, but it remains efficient because documents rarely change.

Interactive apps on the other hand tend to be easier to layout and yet change all the time.

Combining the two just gives the worst of both worlds. In terms of performance, and I also think UX.