4k Video Slow and Out of Sync with Audio

I've got a 2015 15" MacBook Pro with 16GBytes of RAM, and a 1TByte SSD that has a little over 200GBytes free. I’m running Mojave.

I'm trying to play a 4k Video using QuickTime - but the video itself is running slow - the audio is at normal speed and not sync'd up with the video.

The first thing I did was close all browsers and other programs that were open. But, that didn't help. Next, I opened Activity Monitor - the % CPU load was only a few percent. But, RAM usage was very high, 10.5Byte - despite having very little open. Next, I restarted macOS this got the RAM usage to just under 5GBytes - but the 4k video still suffered from this weird slow motion problem - the audio continued to run out of sync with the video. QuickTime isn’t set to play in slow motion - it is set to normal playing speed. Besides, the audio wouldn’t play at normal speed if QuickTime were playing back at some other speed besides the normal speed. Also, the controls of QuickTime have a very long lag. For example, if you press pause while the video is playing to stop the video, QuickTime will take 5 seconds to finally respond and actually stop.

Any idea what I could try next?