The new version now suffers from the same issues as the tvOS version, whereas the 6.1.5 played all files smoothly on iOS 13.
4K HDR files suffer the most, where playback is basically unusable.
But 1080p SDR files do not play smoothly either. It’s as if either the frame rate is low or there is some issue with the sync.
The app is pretty much unusable on iPhone right now.
iPhone XS Max