According to the documentation for libx264 (h264) encoding, we can use a preset of veryfast, faster, fast, medium, slow, etc. https://trac.ffmpeg.org/wiki/Encode/H.264
...for constant quality encoding, you will simply save bitrate by choosing a slower preset.
So while using constant quality, a slower preset doesn't have any affect on video quality, but it can compress the file size smaller.
This theoretically means it should be possible to take a video compressed with preset "veryfast", and recompress it with "slow"- resulting in a smaller file with no loss of video quality.
Has anyone ever done this or know how it can be done?