One unexpected issue is processing time. Each 5K frame is 5120x3840px @ 300dpi (28.1mb), with each of the four reels around 31,000 frames a piece (that’s 123,709 frames @ 3.31TB total). I’m running an i7-7700k @ 4.2gHz CPU with 16GB ram & GTX 1060 6GB GPU, and even then it takes around 26 hours encoding time per reel!
That’s largely due to I/O, make sure you’re encoding to a different hard drive/SSD compared to what you’re reading from. Also if you think that’s bad, try 6464x4852 some time!