I could only get the changed framerate to take effect if the input file was classed as a "raw" file:
ffmpeg -r 5 -f h264 -i input.h264 -vcodec copy -an output.mkv
-f h264 it would default to 25 fps and it could not be changed. Apparently this was because the stream lacked any framerate information at all and this is ffmpeg's default framerate.
Apparently when you use
-r as an output option it duplicates or drops frames so the video plays at the same speed - in this case, not what you want! But changing the input framerate as above will cause the video to speed up or slow down, with no frames lost or duplicated.