Now that the Sony Xperia 1 has been reviewed by more people, there are now sufficient video samples at 960fps to make a judgment on the performance of the slow motion mode. As you may recall, the phone is only capable of recording 0.1 seconds at 960p at full HD 1080p and 0.2 seconds at 720p at the same frame rate. That translates to a maximum of 3.2 seconds at 1080p played back at 30p and 6.4 seconds at 720p 30fps.
There is no increase in recording time from the XZ2 and XZ3 phones which had essentially the same feature as the Xperia 1. We, however, saw a little better color reproduction and slightly better artifact handling on the new phone but so minute an edge that we simply cannot recommend upgrading for this feature or considering it over other 720p slow motion phones like the Galaxy S10 or OnePlus 7.