The just released Xperia XZ3 had a lot of rumors surrounding the slow-motion capability which was to be a cut above previous releases but as first-hand reviewers have shown, the camera module is carried over from the XZ2 and what the phone really changes are the first ever Sony OLED display in HDR, larger battery and a larger screen as main feature changes.
The Slow Motion component is unchanged from the XZ2 at 960fps in Full HD 1080p and 720p with just 6.4 seconds worth of playback at 30fps and 3.2 seconds at 1080p 30fps timeline. The hone only records for 0.1 seconds at 960fps in 1080p and 0.2 sec for 720p. Hardly a usable slow motion format for the people looking to capture long slow motion in the several second time-range.