I am in Shenzhen, trying to download a system image, and I am getting about 20 KB/s. This is presumably because your download server is in Hamburg, Germany. You might consider setting up a mirror site within China for static content (the forums would presumably be all on one server).
I have a unofficial mirror site https://setq.me/rockpi-dl
The files is stored on COS and I need to pay traffic fee, so I only share it in QQ group.
ps. You can verify the integrity of the files using md5sum.
Check Armbian. It has mirrors in China. For images and repository.