>>mongol+f5
No, you're right. SailfishOS inherits the core of the OS from the old Maemo of Nokia N900 fame (though the UI was built from scratch I believe). I tried it back in the day on my Nexus 4 and it was buttery smooth, even with all its fancy animations and gesture-based navigation, which was way ahead of Android at the time.
I always thought SailfishOS would really take off by now, given how advanced and polished it already was at the time, but Jolla's mismanagement nearly jeopardised the whole thing (they filed for bankruptcy last year).