@Kampenuss_twitter That Youtube video looks right to me. OSD is designed to load the lower resolution tiles first. This way you get a smooth "coming into focus" effect rather than a blocky "tiles are popping in" effect. If you prefer the popping look, you can pass immediateRender: true when you create your viewer.
...As for the tile load speed, to some extent that's just your server. One thing you could try, though, would be to use larger tiles. We recommended 256px tiles (254 if you have an overlap of 1) back in the day, but with modern networks, 1024px (or 1022 with an overlap of 1) is probably a better size.
Overlap is in addition to the size, and you want to end up with a power of 2. So if you have overlap of 0 you can use size of 1024. With overlap of 1, the size would need to be 1022 (since the overlap can happen on both the left and right). For overlap of 2, it would be 1020. etc.