Ah I forgot, the original video specs are H264, 5760x2880 at 50/60 megabytes per second, which your rig should be able to handle perfectly fine.
There's no particular tech reasons why they should play worse than any other video encoded by another studio with the same video specs, even using any other encoding software.
As a solution, you might want to try downloading lower resolution versions and see if that makes the framerate improve.