Speeding up and slowing down a clip

Changing a clip's speed is one of the simplest effects in video editing, and one of the easiest to get wrong. Slow motion turns jerky. Sped-up footage with its original sound becomes unlistenable. A timelapse you made by speeding up a long clip ends up the wrong length.

All of it follows from a few facts about frames.

What speed changes do to frames

Video is a sequence of still frames. Most phone and camera footage is 30 frames per second, some 24, some 60.

Speeding up skips frames. At 2×, every second of output uses two seconds of recording, so half the frames are dropped. There are always plenty of frames available, so fast footage looks fine.

Slowing down stretches frames. At 0.5×, every second of output has only half a second of recording to draw from — fifteen real frames to fill thirty slots. The editor either shows each frame twice, which looks like stuttering, or blends between them, which looks smeared.

The fix for slow motion is not in the edit. It is in the recording.

Shoot for slow motion

To slow footage down smoothly, it has to be recorded with more frames than you will show:

• 60 fps footage slows to 0.5× on a 30 fps timeline with every frame real.
• 120 fps slows to 0.25×.
• 240 fps slows to about 0.125×.

On an iPhone, the Slo-mo mode records at 120 or 240 fps. In the Camera settings, Record Video also offers 60 fps.

If you did not record at a high frame rate, keep slow-downs modest — 0.75× or so is usually tolerable — and keep them short.

The audio problem

When you change a clip's speed, the audio has to change with it, and there are two ways to do that:

Change pitch with speed. Like a record played too fast or slow — chipmunk voices at 2×, deep slurred voices at 0.5×. Occasionally funny, usually not what you want.

Keep pitch, change tempo. Speech stays at a normal pitch but is faster or slower. This works within limits — around 1.25× to 1.5× remains listenable — but beyond that, speech becomes hard to follow and artefacts creep in.

For most speed changes, the best answer is neither: mute the clip's own audio and put something else under it. Music under a sped-up section, silence or ambient sound under a slow-motion moment, or a voice-over recorded at normal speed.

Which speed for which job

0.25× to 0.5× — slow motion. For a moment worth seeing closely: a splash, a jump, a product detail. Only with high-frame-rate footage.

0.75× — subtle emphasis. Slightly slower than real life gives footage a considered feel without drawing attention to itself.

1.25× to 1.5× — tightening. Shortens a talking-head clip or a demo without being obvious, especially with the pitch kept.

2× to 4× — skipping time. Waiting for a progress bar, walking from one place to another, a cooking step. The viewer understands time passed without watching all of it.

Beyond 4× — timelapse. For clouds, a sunset, building something over hours. Very high multiples are usually easier to shoot as an actual timelapse — the iPhone's Time-lapse mode does this — than to create from hours of normal video.

Making it look deliberate

Change speed at a cut. A speed change in the middle of continuous action looks like a glitch. Split the clip at a natural break and change the speed of one part.

Keep the rhythm consistent. If you speed up waiting sections, speed them all up by the same amount. Viewers notice inconsistency more than the change itself.

Do not overuse slow motion. One slow moment is emphasis. Five is a music video.

Check the new length. Speed changes alter duration. A 20-second clip at 0.5× becomes 40 seconds, which may push everything after it.

The free options

QuickTime Player can trim and split, but has no speed control for editing — its playback speed buttons only affect how you watch.

Photos can adjust slow-motion sections of iPhone Slo-mo clips, but not change the speed of ordinary footage.

iMovie has speed controls and is free, with templates aimed at home movies.

What Garfi does

Garfi Video Editor is a native Mac video editor with speed control from 0.25× to 4× per clip. Combined with trim and split, that makes it straightforward to change the speed of just one section: split at a natural break, set the speed, and check the new length on the timeline.

For the audio, you can detach a clip's sound, mute it, and lay music or a voice-over underneath instead, with waveforms visible and fades at the edges. Undo and redo cover every step.

Export is MOV or MP4 in H.264 or HEVC, up to 4K when supported, with no watermark. The download is free with an export trial you start yourself, and a one-time purchase unlocks export permanently. There is no account, developer cloud or generative AI.