In my experience, don't use ffmpeg for splitting/join.
MP4Box, is faster and light than ffmpeg. Please tryit.
Eg if you want to split a 1400mb MP4 file into two parts a 700mb you can use the following cmdl:
MP4Box -splits 716800 input.mp4
eg for concatenating two files you can use:
MP4Box -cat file1.mp4 -cat file2.mp4 output.mp4
Or if you need split by time, use -splitx StartTime:EndTime
:
MP4Box -add input.mp4 -splitx 0:15 -new split.mp4