Changing video playback speed sounds simple but involves two separate technical operations that have to stay coordinated. The video track is retimed using a presentation timestamp filter — every frame's scheduled display time is scaled by the inverse of the speed factor, so at 2x speed each frame is shown for half as long. Audio is a different problem entirely: you cannot just play samples back faster, because that would also raise the pitch, turning speech into the familiar "chipmunk" effect. Proper speed changes need a time-stretching algorithm that adjusts tempo independently of pitch.
This tool handles both halves correctly. The video stream is retimed using FFmpeg's setpts filter, and the audio stream is processed through the atempo filter, which performs genuine tempo-shifting rather than simple resampling — voices and music speed up or slow down without turning into a squeak or a drone. Because atempo's single-stage range is limited to between 0.5x and 2.0x, more extreme speed changes (3x or 4x) are achieved by chaining multiple atempo stages together, and the tool handles that chaining automatically so you can pick any speed from a quarter-speed crawl to four times normal without worrying about the underlying filter math.
Pair it with Trim Video to cut down a clip before speeding it up, or Compress Video afterward to shrink the result.
