I would suggest taking a look at Python + MoviePy (MoviePy downloads and installes FFMPEG for you the first time that you use it).
- Cost $0.00 - all free, gratis & open source software.
- Frills none - not even a GUI
- Power Lots thanks to FFMPEG
- Ease of use - quite high.
- Size Tiny - FFMPEG installed < 150 MB, Python installer < 34 MB, MoviePy + dependencies 12 MB
The process would be to make a note of the exact times of each section and whether each is a speed-up or slow-down, and by how much, in the example below I have done a factor of 5 for both.
Then in python terminal, (or ipython is better), you can do the following:
from moviepy.editor import *
clips =  # Storage for the clips
master = VideoFileClip("name_of_input_file.mp4") # Edit name to real one
clips.append(master.subclip(0, 20).speedx(5)) # First 20 seconds at x5
clips.append(master.subclip(20, 25).speedx(0.2) # Next 5 at 1/5 speed
#clips.append(... # However many others you need
final = concatenate_videoclips(clips) # Add them all together
final.write_videofile("timeplay.mp4") # Save to a file go and have a coffee
Obviously you can skip &/or repeat sections, etc. and there are a lot of other cool tricks that you can do with MoviePy.