Make a slow-motion video

That depends on the frames you filmed. At 0.5× a clip plays at half its frame rate, so 60 fps footage becomes 30 fps, and 120 fps footage at 0.25× is 30 fps too. Below 30 fps the tool holds each frame longer rather than inventing new ones, so a 30 fps clip at 0.25× shows each frame four times, and the motion steps.

It slows down with the picture and keeps its pitch, stretched by SoundTouch, so a voice stays a voice rather than dropping an octave. Untick Keep the sound for a silent clip.

It's stretched or squeezed to the new length and keeps its pitch, by SoundTouch, an open-source time-stretcher that runs in your browser. Untick Keep the sound to leave it out instead.

As smooth as the frames you filmed. Slowing down uses every frame the camera recorded, and the tool never invents frames in between. A 60 fps clip at 0.5× plays every frame at 30 fps; a 30 fps clip at 0.5× shows each frame twice. Film at 60 or 120 fps if you want slow motion to stay smooth.

Sped up, the same as the original, with frames left out: a 30 fps clip at 2× keeps every other frame. Slowed down, every recorded frame at its new pace, and never below 30 fps if the original had 30 or more. Never above 60 fps.

An MP4 with H.264 video and AAC sound, whatever you dropped. The picture is re-encoded, so it works on videos up to 15 minutes long, before and after the change.

No. It's decoded, retimed and encoded in your browser, on your device.