FLV

FLV existed because Flash needed something it could stream over an ordinary web server and start playing before it arrived. Its structure is a simple sequence of tagged chunks — video here, audio there, metadata occasionally — with no requirement that the file be complete or that anything be indexed in advance.

For the era, this was the right answer, and it is why practically every video site between 2003 and 2013 served FLV. What remains is an enormous archive: tutorials, lectures, recordings, personal collections, all in a container nothing modern will open.

The good news is that most of that archive is younger than 2008, and files from then on almost always hold H.264 video and AAC audio — precisely the streams an MP4 accepts.

When the streams inside a container are already ones the target container accepts, converting means unwrapping and rewrapping — reading the video and audio out and writing them into a new file without touching a single frame. That is remuxing, and it runs at whatever speed the disk manages. An hour-long FLV becomes an MP4 in a few seconds.

The exception is the older files. Sorenson Spark and VP6 video, or Nellymoser audio, have no place in an MP4 and must genuinely be re-encoded — slower, and with a quality cost. A quick way to tell which you have: if the file is from before about 2008 or looks like early web video, expect the slow path.

Or skip the question. A player with its own decoding stack opens FLV as it is, VP6 and all, and converts nothing.