This comment is seemingly a reaction to the post's title and doesn't address (or even acknowledge) the contents of article. Do you disagree with the technical rationale the author has presented?
The content of the article does not really make a technical argument. It just says "XUL can not be fast, because I said so". The code of Firefox has always been a historical grown mess with a lot of loose ends that need to be entangled. I'm convinced that a XUL version can be made fast as well if you spend the appropriate amount of engineering hours on it.