Quicktime playback is choppy / stuttering on powerful iMac Pro?

Hello

I'm suffering playing back a 4K .mp4 file. It's 1GB in size, 16 seconds long, and the playback is choppy in Quicktime and VLC. The footage is playing from the iMac's internal HD.

I have an iMac Pro (2017) with the following specs:

3.2 GHz 8-Core Intel Xeon W

32 GB 2666 MHz DDR4

Radeon Pro Vega 56 8 GB

I'm very surprised it's choppy - this is a powerful machine. What could it be?