@buggyyang - Glad you found it useful, and I moved this conversation to Discussions as it is not an ffmpegio issue.

I assume the -r in ffmpegio here performs differently to ffmpeg command-line?

Yes and no. Yes, they are different but no because you are invoking the difference.

When calling FFmpeg, the option order matters.

With ffmpeg -r 24 -i test_video.mp4 -, you're forcing the input to have 24 fps, i.e., play back in slow-mo as you described, because -r here is an input stream option.

If you want to change the framerate, you need to use the output -r option: ffmpeg -i test_video.mp4 -r 24 -

In ffmpegio, if you give an ffmpeg option name (minus the leading dash -) as your keyword argument then you are defining ffmpeg output options. By appending _in to the ffmpeg option name, your keyword argument turns into an ffmpeg input option. Using ffmpegio.transcode as an example,

ffmpegio.transcode('test_video.mp4','low_fps.mp4',r=24) # equivalent to ffmpeg -i test_video.mp4 -r 24 -
ffmpegio.transcode('test_video.mp4','slow_mo.mp4',r_in=24) # equivalent to ffmpeg -r 24 -i test_video.mp4 -

This applies everywhere including ffmpegio.open. Notice that your first call videostream opens with the new output rate (thus dropping excess frames) while the second call videoout with its input rate, which matches the videostream framerate.

Hope this makes sense to you. Also, if you're only transcoding, use the ffmpegio.transcode as I show above as it's a whole lot faster than the dual-stream for loop solution you have currently.