FFMPEG Conact Demuxer Re-Encoding When Trying to Merge MP4s

Question

I am trying to merge MP4 recordings straight out of OBS together into one MP4. From what I understand, I am using the concat demuxer to achieve this as it merges the files "extremely quickly" without re-encoding.

I am using the command

ffmpeg.exe -f concat -safe 0 -i "mylist.txt" -c copy video.mp4

as explained in this post

2. concat demuxer Use this method when you want to avoid a re-encode and your format does not support file-level concatenation (most files used by general users do not support file-level concatenation).

The command runs successfully, but the process takes a very long time. I assume the videos are being re-encoded as this is what shows:

Output #0, mp4, to 'video.mp4': Metadata: encoder : Lavf58.51.100 Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x2160 [SAR 1:1 DAR 8:9], q=2-31, 1

I'm unsure as to why the process is taking so long and want to know how I can merge the MP4 files into a single MP4 output file almost instantly.