@fujiwara you have a point in that it might be much more expensive to improve the streaming quality. Perhaps, @dtube has more important priorities right now compared to this (I am not sure myself). HD options would be great too since most users are accustomed to the HD video quality on Youtube, Twitch etc.
I believe that the related videos currently displayed on your DTube UI is dependent on the accounts that you've followed. Personally, I only see videos from users that I've followed but not because it's relevant to the video that I'm currently watching. If @dtube can build a system that displays relevant videos to the ones users are currently watching, this can help to create more exposure for dtubers whilst keeping users engaged on the platform for a longer duration.
I have a pretty beefy Internet connection where I live and I'm experiencing the same things, videos aren't watchable when selecting the source option as they stutter way too much.