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Slow-motion videos are really just videos recorded with a very high frame rate.

Say your video was recorded at 120fps and your project is 30fps.

Select your video in the timeline, and choose "Automatic Speed" and it will play every frame, resulting in a 25% speed, with perfect results, since it will use every frame recorded by the phone.

The clips are high frame rate not slow motion. Put the clip in the project and in the retiming menu select Automatic