Poor video playback performance, solutions?

We are using Unreal’s built in video players to show full-screen videos to the player, but some players are reporting poor performance: stuttering.

They are running lower-spec PCs, but the raw .mp4 files run fine through VLC.

The scene we are running the video in has nothing else running behind it, so AFAIK it’s not a case of other things competing for CPU.

Other than dropping the resolution, is there anything else I can do to improve the performance of video playback?