Thanks for the info! It was a failing drive.
I moved the parity drive to the array and tried copying to it. The transfer speed started at ~200MB/s but slowed to ~40MB/s within a minute. I put a different 2TB drive in, copied all my files to it, then made the good 12TB drive the parity. Now have 200MB/s parity sync
Thanks for the help! The drive has been RMA'd