Double the playback speed
speed & reverse
// what it does
setpts=0.5*PTS multiplies every frame's presentation timestamp by 0.5, so the whole clip plays back in half the time — 2x speed. atempo=2.0 doubles the audio tempo without shifting pitch, keeping voices natural rather than chipmunked. Both streams are re-encoded; setpts keeps every frame (nothing is dropped), it just packs them into a shorter timeline.
// shell
$ ffmpeg -i input.mp4 -vf "setpts=0.5*PTS" -af "atempo=2.0" output.mp4// gotcha
Keep atempo the reciprocal of the setpts multiplier (0.5*PTS pairs with atempo=2.0) or the audio and video drift apart. For faster than 2x, modern ffmpeg (3.4+) accepts a single atempo up to 100.0, so atempo=4.0 is valid — chaining (-af "atempo=2.0,atempo=2.0") is the older idiom from when 2.0 was the hard cap and still works. The lower limit is still 0.5.