QML runs interpreted at runtime and Qt is very close to native C++, so both are actually quite lean and fast on these devices — the overhead is surprisingly small compared to full Android Wear, and we've tuned it heavily over the years to keep battery and UI responsiveness solid. If you see devices lagging, like the sawfish (Huawei Watch 2), its usually due to updating the android base to a newer one as the manufacturer planned for and losing some of those optimizations in the process.