When you film in slow motion, all you're actually doing is increasing the frame rate at which you film. So if you normally record a 10 second video at 30FPS, to achieve Slow Motion you're increasing the frame rate to 60FPS (for example) for that 10 seconds.
The thing is, when you go to play back this clip, there's no way for your player to magically know if you're trying to play back a slow motion video or if you're trying to play back a high frame rate video (a 60FPS video playing for 10 seconds will look sharper and clearer than a 30FPS video playing for 10 seconds).
So to achieve slow motion you need to take your 10 second 60FPS video, put it into a video editor and reduce the speed by 50%. This will result in a 30FPS video that now lasts 20 seconds (1/2 speed slow motion). So Filming a 10 second clip at 120FPS and slowing it down by 1/4 will result in a 30FPS video that lasts 40 seconds (1/4 slow motion)
You should be able to easily do this in Quik.
