Many people have complained over the years about choppy video playback, and there haven't been any solutions posted that reliably work. Playback on my Macbook Pro 15" mid-2015 is sometimes fine, sometimes choppy.
I don't bother trying to use LR to play cataloged video any more. Instead, I use the Quicktime player. You can right-click a video, choose Show In Finder, and then double-click the video to launch Quicktime. A little klutzy and inconvenient. (I use the Open command of my Any File plugin, which does that with a single keyboard shortcut.)
OK. I have to accept this, but with LR 5 I did not have the problem. I "upgraded" to LR 6 and now I can not play video out of LR. What a pitty.
I am having the same problem every since Lightroom 6.7 (2015.7). Video playback is INSTANT and without any problem in Lightroom 6.6.1, and versions since are just plain horrible, with delays, stuttering, and when I stop playback Lightroom needs a couple of seconds to "recover itself".
Quicktime is not an option - I'm on a Windows system and QT isn't actively developed for Windows anymore (in fact, it is recommended to uninstall it because it has too many unfixed security issues).
Something quite significant must have changed under the hood between LR 6.6.1 and LR 6.7. It doesn't affect just video, but that's a different story (and I'm still using LR 6.6.1 for that reason - remind me again why I pay for the CC subscription every month...?)