The browser engine has been updated so that's already worth an update by itself.

That sounds a lot like the tablet, which also suffers from awful UI performance. Maybe it's a Qt bug that only shows itself on large resolutions? In any case, I've noticed that all parts of Sailfish will stutter to no end, but Android applications couldn't run smoother. If you pull up the Sailfish launcher on top of a running Android application, it'll be as smooth as it gets, but if you pull it up on a running Sailfish (Qt) application or any other part of Sailfish, it'll stutter and jerk like you wouldn't believe. Maybe the next Qt update will fix it, because it's obvious that it shouldn't stutter at all. The Jolla phone is more than five years old and - unless it runs out of RAM - the UI is perfectly smooth.