I never paid any attention to mirror selection when in Europe nor willlingly selected this particular mirror (as it happened, it just worked reasonably before).
Good to know they are ranked by speed (sort of). But, the download speed obtained varies according to the server, to the location (or IP) of the user and may change over time. So this ranking is at best an advice.
I use a provided bash script to select mirrors by speed with Arch. Maybe this kind of tool could be adapted to Koreader?
https://wiki.archlinux.org/index.php...#List_by_speed
https://www.archlinux.org/mirrorlist/
The other idea would be to find a Chinese host since Koreader is now a world class application.