Earlier quoted context omitted.
Still no answers but a lot of malicious downvotes.
Allow me to somewhat quippishly rephrase your original post: "I've never heard of badly performing cross-platform apps, except for the ones written in what's currently the most popular cross-platform toolkit and the ones written in one of the most popular cross-platform toolkits a generation ago."
Qt, Wx, Java, and many others don't or no longer perform poorly. So why the surprise there is one more? Maybe some folks don't know they exist.