"The 2D layout in HTML is slow because the DOM (with the millions of conflicting hacks on top of it) is slow, not the other way around."

Well yeah, all those hacks of html that we cannot get rid of, because of backwards compatibility are probably the main reason the DOM is slow. HTML was made to view documents after all and not design UI's. And now it is, what it is. But there are options now!

So yes, it is definitely possible to build snappy 2D layouts. I build one with HTML, using only a subset and that worked out somewhat allright .. but now I am switching to WebGL. And there is world in between in terms of performance.