It's true in theory, but often the actual implementations make the point moot.

From the article, it looks like the optimization effort wouldn't go to XUL.

> Nobody was realistically touching XUL layout code if they could avoid it. Part of it was just a matter of priority (we’d rather make the web faster for example).

We're hitting the same situation with Javascript, which in theory should be slower than most static languages, yet the sheer amount of work and optimizations done to its engines make it faster in real world tests.