Apple has released a preview video showing the video capabilities of the iPhone Xs in 4k 30, 4k 60 and slow motion 1080p 240fps. You can see in the liquid and sound part of the demo the quality delivered by the full HD slow motion but it is hard to judge per pixel sharpness and or artifacts from such a short high shutter video. We will need more real-world samples in order to see if there has been an improvement from the iPhone X of 2017.
We have isolated a part of the footage to see the slow-motion compression and we could detect aliasing in the edges, softness and compression artifacts which are typical of mobile phone high frame rates. The iPhone Xs seems to still have them to some degree. Color and motion on the codec seem to be excellent especially in 4k which looks extremely detailed and with more than acceptable dynamic range.