You can see it rendering its buttons and menus.. slowly. Typing may take 10 seconds to respond. It is completely unoptimized for ARM, or at least was when I tried it.
It's true it can't use libraries like the Intel Performance Primitives on ARM, but that's not why the UI is slow on RPi. RPi's UI is slow because all graphics-intensive GUI stuff on RPi is slow. Firefox is unusable, for example.
