Change video speed

How Video Speed Changer works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

Two clocks, one file

Changing speed means scaling two timelines. Video is retimed with setpts, which multiplies each frame's presentation timestamp: 2× speed halves every timestamp, so the clip finishes in half the time.

Audio cannot be resampled that way without changing pitch, so the atempo filter time-stretches it instead. It only accepts 0.5× to 2× per instance, which is why 4× is atempo=2,atempo=2 chained.

What you get and what you lose

At 2× a five-minute clip becomes two and a half minutes with the audio still matching. At 0.5× it doubles — the picture repeats existing frames, so a fast pan looks stepped rather than smooth.

True slow motion comes from recording at 120 fps at capture time. No amount of processing creates frames that were never recorded; the page says so instead of pretending.

Where it is genuinely useful

Lecture recordings watched at 2×, screen recordings of long processes compressed into a minute, dashcam footage reviewed quickly, and a recipe clip turned into a fast demo.

All of those are local edits of a file you already own — no upload, no account, no watermark.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions