@morbelek Thank you very much for the input, it set me on the right path!
TLDR: For me, this is an IPv4 issue and with both (IPv4 and IPv6) enabled and properly working, things are back to normal right now. For both websites, j-novel.club and jnc-nina.eu, webreader pages load within 500 ms, downloads have >3MB/s and site-navigation is as any other.
I must have made a slip-up somewhere because the IPv6 DHCP server in the router was not configured properly.
The longer story: After reading about the issue with my ISP Telekom and cloudflare some more, I visited my neighbours who also use Telekom. To my surprise everything was working fine over there (used the laptop with USB booted manjaro). I noticed that Firefox shows an IPv6 address for m11.j-novel.club but on my side it always showed an IPv4 address, even with IPv6 enabled. After careful comparison between the router settings I finally noticed that the IPv6 DHCP server in my router was missing a vital checkmark:
After a restart of the router things sped up a lot and Firefox is now showing me an IPv6 address for m11.j-novel.club while using the webreader.
In the end it was two things:
The IPv4 only config I was using for the last couple of years suddenly stopped working properly (Likely due to the Telekom - cloudflare issue that is apparently known).
I made a mistake with the IPv6 config last week.
If this or a similar issue comes up again I'll get back to it, but for now I consider the issue resolved.
Thanks again for the input and I hope this may help others with similar issues.