FFMPEG slow / fast motion part a video anywhere
This slows down one part of a video and keeps the rest as is.

ffmpeg -i Soon.mp4
  -filter_complex
     "[0:v]trim=0:2,setpts=PTS-STARTPTS[v1];
      [0:v]trim=2:5,setpts=2*(PTS-STARTPTS)[v2];
      [0:v]trim=5,setpts=PTS-STARTPTS[v3];
      [0:a]atrim=0:2,asetpts=PTS-STARTPTS[a1];
      [0:a]atrim=2:5,asetpts=PTS-STARTPTS,atempo=0.5[a2];
      [0:a]atrim=5,asetpts=PTS-STARTPTS[a3];
      [v1][a1][v2][a2][v3][a3]concat=n=3:v=1:a=1"
  -preset superfast -profile:v baseline output.mp4


How does it work?


The trim and atrim filters cut the video into different parts, from 0–2 seconds, from 2–5, and from 5 to the end.
In each part a setpts/asetpts filter is applied, which, when using the option PTS-STARTPTS, "resets" the presentation timestamps of each frame in each part, so that they can later be concatenated easily.
Each part is given an output label (e.g. [v1] through [v3]).
For the video parts that need to be slowed down, every presentation timestamp is doubled (2*(…)), which effectively halves the speed of the video. If you wanted to speed up the video, use a multiplier lower than 1.
For audio parts to be sped up / slowed down, use the atempo filter, whose parameter is the speedup (e.g., 0.5 = half speed).
These parts are finally concatenated using the concat filter.