If all you want to do is merge all of the files from one long take into just one file you can use "ffmpeg". The process is extremely fast as ffmpeg is only concating the clips to just one file, no processing is performed. GoPro also splits long video clips into several chapters (files) like DJI does. Here is the command I used to merge three GoPro files into one.
ffmpeg -i concat:"GOPR6076.MP4|GR016076.MP4|GP026076.MP4" -c copy GOPR6076_merged.MP4
where GOPR6076.MP4 is the first file,
GR016076.MP4 is the second file and
GP026076.MP4 is the third file
Order is important, so you will want to add the filenames in the order they were recorded.
OK, I ran a test and a change will need to be made to how the list of files to be concatenated are passed to "ffmpeg". The command is -
ffmpeg -f concat -i DJIMerge.txt -c copy CompleteVideo.mp4
The contents of the file "DJIMerge.txt" contains the list of files to be concatenated in the format of
file 'DJI_0001.mp4'
with one entry per line.
"CompleteVideo.mp4" can be any name you want to give to the merged video files output.
For example, in the test I performed the contents for the file DJIMerge.txt contained -
file '20201119-DJI_0004.mp4'
file '20201119-DJI_0005.mp4'
file '20201119-DJI_0006.mp4'
The total # of bytes for the three files added up to - 7,915,728,547
The byte count for the merged video file is 7,915,774,235
Elapsed time to produce - 3 min 15 secs, source and destination files on the same drive
1 min 35 secs, source and destination files on different drives