Jan said:

By looking at the encoding settings (MediaInfo can do that) it is obvious that encoding speed has been a major goal, which comes as no surprise considering this is just a workprint. However, the settings could have been significantly optimised to achieve a better quality with less blocking and yet without a drastic slowdown. F.e. encoding with 1-pass constant bitrate as you did is much inferior to CRF mode quality-wise.

I haven't seen the preview but I'd just like to echo Jan's suggestion. CRF mode is designed specifically to be fast, efficient and high quality. If you're not aware of it, basically you set a quality level, with lower numbers being better, and CRF determines the lowest bitrate that can achieve that quality. Talking heads? Low bitrate. Explosions in the middle of a hurricane? High bitrate. It happens automatically and quickly, with only one pass. I've had great luck with it.

Keep up the great work! :)