Per the browser performance testing I did on the Jolla & Nexus5, FireFox running inside Alien Dalvik VM on the Jolla is faster than any browser on the Nexus5 running Sailfish (SD800 is miles faster than SD400 in Jolla). This shouldn't happen.
I tested UP performance on the Nexus5 too ... it also uses Qt, QML and libhybris for hardware adaption. If there are inherent issues with them, then they'd suffer too.