How to Slow Down a Video on Mac

The fastest way to slow down a video on Mac is to select the clip in an iMovie timeline, click the Speed button, and choose Slow from the Speed pop-up menu. Here's the full control set, and how to tell whether the slowed result will hold up before you build an edit around it.

Slow down a video on Mac with iMovie

Select the clip and click the Speed button in the adjustments bar. Choose Slow from the pop-up menu. It's set to 50% by default, meaning half normal speed. iMovie marks the clip with a small turtle icon, which is a genuinely useful bit of feedback once a timeline has a dozen shots on it and you've lost track of which ones you touched.

For a specific moment rather than a whole shot, select a range inside the clip first. iMovie puts the speed sliders above the selected range, so the slowdown applies there. The Smooth checkbox is worth turning on whenever you do this: it eases the clip into and out of the slow section instead of snapping between rates, which is what makes a slow-motion beat feel intentional rather than like a dropped frame.

If you want a value other than 50%, Custom lets you set it directly.

Why slowed footage stutters, and how to see it coming

Slowing a clip doesn't create new pictures. It takes the frames that were recorded and spreads them across more time, which means each one has to stay on screen longer. Footage shot at a high frame rate has plenty in reserve and absorbs this easily. Ordinary footage doesn't, and past a certain point the eye starts seeing individual frames being held rather than continuous movement.

There's a precise way to check rather than squinting at a preview. Open the clip in QuickTime Player and press the Right Arrow key, which stops playback and advances exactly one frame. Left Arrow goes back one. Step through the slowed section and count: if the picture is identical across several presses, those are held frames, and that's the stutter you were seeing. Option-Left Arrow and Option-Right Arrow jump to the start and end of the movie when you want to get back out quickly.

Once you've seen how many frames are being repeated, the decision usually makes itself. 50% might judder where 75% doesn't.

Word-level timing, taken from the audio itself

Montaj doesn't slow footage down; that's iMovie's speed control and there's no reason to look elsewhere for it. What it handles is what comes after, and one detail matters here specifically: its subtitles carry word-level timing taken from the audio itself, so each word is pinned to the moment it's actually said rather than to a caption block that was guessed at.

Every export is verified before it's handed over. Duration, dimensions and audio are checked, and frames are decoded to confirm the file plays. On a clip whose runtime has changed, duration is exactly the thing you'd otherwise only notice later. Use iMovie to slow down a video on Mac, and let the checking of what came out of it stop being your problem.