Can't play back any h.264 video files in Lightroom CC 2015.12 smoothly in High Sierra - sound plays fine, but video pauses frequently, jerkily catches up, then pauses again, over and over with every video file. Worked fine on this machine (2017 4k iMac 3GHz) under Sierra.
This is still an issue with the latest updates. It's so bad that, while I still keep videos in my LR catalog to associate them with photos I took around the same time, I've exported all of my videos to Apple Photos so I can reliably watch them.
In general, LR's support for video is mediocre at best. Over the years, lots of people have reported in the forums problems with playback, without authoritative solutions posted.
An alternative to using Apple Photos to play your cataloged videos: Right-click the video thumbnail in the catalog and do Show In Finder. Then double-click the video file in the Finder to play it in the QuickTime player.
You can make that more convenient with the Any File plugin, which provides an Open menu command that will open any cataloged item in the default application assigned to that item. You can assign a keyboard shortcut to Open, e.g. Ctrl + O, to let you more quickly open any cataloged video within QuickTime.