Apple TV 4K Problems

I recently took some nice photographs on a trip to Norway. I put together a 4K slideshow in FotoMagico 5 and output it via the Share menu to Apple TV, which creates a .m4v file that is placed in iTunes. This plays fine on the display of a Mac Pro running iTunes. When I try to play it on my big screen TV via Apple TV 4K using Home Sharing, it stutters or freezes.

The connections are hard-wired over a Gigabit ethernet network. I’ve had no difficulties playing 4K commercial movies on the Apple TV 4K or from a 4K Blu-ray player. HDMI cables are all premium certified.

If I use FotoMagico to output the same slideshow to YouTube, also in 4K, it plays without a hitch on the TV.

I understand that the Apple TV streams 4K video but does not download 4K. Does this limitation also apply to 4K over the network? Or can the settings of FotoMagico be tweaked?