Speed Change makes a video or an audio file faster or slower as a whole: watch a recorded course at 1.5x and keep it that way, replay sports in slow motion, squeeze a 30-second voiceover into 15 seconds. It is the “Speed” tab of the audio/video workbench, with a range from 0.25x to 4x, a slider step of 0.05 and shortcut buttons for 0.25 / 0.5 / 0.75 / 1.25 / 1.5 / 2 / 3 / 4.

Sound goes through ffmpeg's atempo filter, which changes speed without touching pitch — 2x speech keeps its voice instead of turning into a chipmunk. Since one atempo stage only allows 0.5x–2x, the tool cascades them automatically: 4x becomes atempo=2,atempo=2. The picture uses setpts=PTS divided by the rate to rewrite timestamps, staying aligned with the audio.

1. 01Add a file and select it— video or audio both work; switch to the “Speed” tab.
2. 02Set the rate— click a shortcut or drag the “Rate” slider (double-click it to return to 1x). The panel below shows the expected output duration, for example a 10-minute video at 2x becomes 05:00.
3. 03Check the estimated command— you can see the exact atempo cascade and setpts expression; the button is disabled at 1x.
4. 04Run it— click “Start speed change” and download <original-name>_2x.mp4 (audio files keep their extension, for example _1.5x.mp3).

Rate range

— 0.25x–4x, values outside are clamped; there is no segmented or gradually changing speed curve.

Video output is fixed

— whatever the source container, MOV or MKV, video is always re-encoded to MP4 (H.264 CRF 21 + AAC 128 kbps); AV1 sources cannot be decoded.

Slow motion does not add frames

— below 0.5x the picture just repeats existing frames instead of interpolating new ones, so movement visibly steps.

Audio quality

— above 2x or below 0.5x, atempo adds a slight warble and phasey quality; speech tolerates it better than music.

What it does not do

— it does not change pitch (for key changes use the pitch subtab of “Audio trim and merge”), does not reverse (use “Audio reverse”) and does not trim a time range.

Speed changes run locally in the browser through ffmpeg.wasm and files are never uploaded. On first use the engine, about 31 MB, is downloaded from a CDN (the site asset domain or jsDelivr); that request contains neither your audio nor your video.