ffmpeg -c copy vs re-encoding: speed, quality and limits

-c copy (stream copy) moves the compressed audio and video packets into a new container without decoding them: it takes seconds and loses nothing, but you cannot scale, filter or change the codec, and cuts can only start at keyframes. Re-encoding decodes and compresses again: slower and lossy, but it allows any filter, codec or frame-accurate cut.

Criterion by criterion

Same task both ways

ffmpeg -hide_banner -loglevel error -f lavfi -i testsrc2=size=1280x720:rate=30:duration=10 -c:v libx264 -preset veryfast -g 60 -pix_fmt yuv420p -y src.mp4
start=$(date +%s%N); ffmpeg -hide_banner -loglevel error -i src.mp4 -c copy -y copy.mkv; t_copy=$(( ($(date +%s%N) - start) / 1000000 ))
start=$(date +%s%N); ffmpeg -hide_banner -loglevel error -i src.mp4 -c:v libx264 -preset medium -crf 23 -y reenc.mkv; t_enc=$(( ($(date +%s%N) - start) / 1000000 ))
echo "stream copy: $( [ "$t_copy" -lt "$t_enc" ] && echo faster || echo slower ) than re-encoding"
for f in copy.mkv reenc.mkv; do printf '%-9s codec=%s\n' "$f" "$(ffprobe -v error -select_streams v -show_entries stream=codec_name -of csv=p=0 $f)"; done

stream copy: faster than re-encoding
copy.mkv  codec=h264
reenc.mkv codec=h264

Timings vary from machine to machine, so only the comparison is shown. On this 10-second 720p clip stream copy is near-instant while libx264 has to encode all 300 frames.

When to use which

• Copy: change MP4 ↔ MKV, drop or extract a track, trim at keyframes, add -movflags +faststart.
• Re-encode: resize, burn subtitles, change codec for compatibility, cut at an exact frame, reduce file size.
• Mix them: -c:v copy -c:a aac copies the video and converts only the audio.