My Samsung S9 can produce nice slow-motion with 120fps. However, on the phone the videos are stored in 120fps, recoded to 29.99fps - so that you can view the slow-motion directly. But sometimes you also want the original video back. Here is a quick script to recode the video to normal speed:

ffmpeg -i "$FILENAME" -vn -acodec pcm_s16le -ar 44100 -ac 2 output.wav
sox output.wav fixed.wav speed 8.0
ffmpeg -i "$FILENAME" -vf setpts=1/8*PTS -an -r 30 -crf 18 output.mp4
ffmpeg -i output.mp4 -i fixed.wav -c:v copy -c:a aac realtime.mp4
rm output.wav output.mp4 fixed.wav

Unfortunately, you can not use ffmpeg for the audio too... But the trick using sox works fine!