[FFmpeg-user] Why is concat so slow on a powerful machine?

[Kevin Duffey]

Hi all,
I understand that the ffmpeg concat will take a series of files, concat them into one file, and update the header info. In my case, I am concating 2 to 4 DNxHR SQ videos into one. I am on a quad core i7, 3.8Ghz, 64GB RAM, full NVMe SSD. Nothing else running. Still, it seems that to concat a combo of 2 files making up a total of 1.2 hours, it will take 20+ hour to complete. This seems incredibly slow to me. As I can redner with FX in real time on this machine, and concatting is basically putting the bytes of one file after the other (in a new file) while updating the header details for the length and such, it seems like this process should take minutes. Mind you I am using the -c copy option, so no re-render is happening. I would think it would start off with the first file, copy it to a new file which should be super fast, then append the video (or audio) data, which again should be very fast as it is mostly just copying the data stream. Then adjust the header and be done. 
What am I missing? Is there something I should be doing with threads, GPU, etc to help speed this up? 
Thanks.