Picture without quality loss
A video file stores every frame with a time stamp. To change the speed, the tool gives each frame a new time stamp and copies the frames as they are, so the picture keeps its full quality and even a long video is done quickly. A 30 frames per second video slowed to 0.5x plays at 15 frames per second, so it looks the same, only slower.
When speeding up would push the video above about 60 frames per second (for example 4x on a 30 fps video), most players could not show all those frames anyway. In that case extra frames are dropped and the picture is encoded again at 30 or 60 frames per second. The note under the options tells you which of the two will happen.
Natural sound at any speed
Playing sound faster normally raises its pitch, and voices start to sound like chipmunks. With Keep, natural pitch the tool cuts the sound into very short pieces and puts them together at the new speed, choosing each piece so that the waves line up. Speech stays clear and music keeps its key. At extreme speeds such as 0.25x or 4x a slight echo or roughness can be heard, which is normal for this method.
Keep, pitch changes plays the sound faster or slower like a tape or a record, so pitch follows the speed. Remove leaves the video silent, which is useful for time lapse clips.
Formats and limits
MP4, MOV, WebM and MKV videos are supported, and the result keeps the format of the original. MP4 and MOV get AAC sound so they play on phones, Windows and Mac. Everything happens on your device, so very long videos take longer and need more memory on older phones. Firefox cannot create AAC sound: for MP4 and MOV with sound, use Chrome, Edge or Safari, or choose Remove.