mp4 / mkv / webm / mov / m4v are remuxed instantly. avi, flv, ts, mpg, wmv, ogv, 3gp are re-encoded with ffmpeg — much slower, avoid for 4K.
Slow speed is usually YouTube throttling + file size (a 10-min 4K video can be 3–5 GB). 8 parallel connections are now used automatically.
Great for editors: grab the pure 4K / 1440p / 1080p video-only stream.
If a codec is chosen above, it overrides the original stream. Metadata is embedded automatically.
The file downloads through your browser once processing finishes.