Really? Nokia N9/Meego Harmattan was also X11-based, and was much quicker than other dual-core androids
MAEMO for N900 was total mess being mix of different libraries inclusing GTK and Qt, so it needed more memory requirement on such memory-limited device.
N9's Harmattan was best-ever, state of the art mobile operating system. The only problem was that, it was announced dead even before n9 retailing, but still selling well(and better than Lumia, that was announced as supported in long-term replacement for it)