I snapped four MTS files of 2.1 GB each into one film of approx. 64 minutes. The Rendering time was 7~8 hours and the result is useless! The length is correct but people in the movie move like in old movies, that is they seem to stand still for some frames and then move. The sound is incomprehensible and sound like playing by fast forwarding.

Any suggestions of what might be the problem and how to overcome it?

Check the dependencies melt and ffmpeg. Both should be up to date. Ubuntuish distros seem to be prone for this cause of trouble. You may also set an in and out point in your timeline and render that part to check. If you simply want to concatenate the four MTS files, check this page for lossless and superfast concatenation: https://trac.ffmpeg.org/wiki/Concatenate

Maybe I was a bit fast there. I'm new here so I have to get to know the forum and it's function. I accepted your answer, capslock, but I actually haven't tried using your suggestions. Thanks anyway. I'll post more when I have tried.