60fps at 1080p is possible
Some old combinations of firmware and DJI GO / Litchi allowed 48 & 60fps at 1080p, and I remember people saying that this video modes were even listed on some official documentation, but the P3S currently supports only the following modes:
- 2.7K: 2704 x1520p 24/25/30 (29.97)
- FHD: 1920x1080p 24/25/30
- HD: 1280x720p 24/25/30/48/50/60
As for the OP question, you can slow-down any video in post-processing, but - as others as said - the best results are achieved when you start from a source at a high framerate. For example, if you have a clip at 60fps and you're producing a 30fps video, that clip can be slowed down 50% and still look good (30/60=0.5) as no frame will need to be duplicated. With source at 120fps and target at 30, you can go as low as 25% (30/120=0.25).
On the P3S, your choices are limited: record at 30fps (2.7K or 1080p) and have good resolution but minimal ability to do slow-motion (you can still try, but at some points movements will look jerky as each frame will be duplicated too many times), or record at 60fps (720p) and accept mediocre resolution but smooth movements down to 50% speed.
Lastly, keep in mind that if you shoot at 60fps, the camera has maximum 1/60s to capture each frame. If there is not enough light (i.e. dusk, dawn, cloudy), the P3S will start bumping up the ISO in order to stay below 1/60s, and video quality will degrade. In other words, for high framerates, you need plenty of light.