[solved - IPv4 issue] Slow website/download for several weeks

Eleusis

The website (j-novel.club), the forum, downloads and webreader are slow or take very long time to load. The same is true for the JNC-Nina website (jnc-nina.eu). The issue has also been reported by others on the german Discord section. While most describe it as "sometimes", for me it is continuous and only with these two websites (which resolve to the same IP). To investigate I've set up a fresh manjaro-kde-24.1.2 that boots from USB. Firefox is preinstalled without addons and when I visit JNC, this is what I see:

The <15 kb thumbnails take over 10,000 ms to load and navigation through the site is equally slow.

When I try to open one of the free Manga chapters it loads the first few pages for 30-120 seconds and then stops (doesn't matter which Manga):

If I reload, the images are always loaded starting at the first page. Even with abundant patience, reading an entire chapter of a manga is practically impossible.

On my normal system I often have over 100,000 ms for a single ~500 kb image:

When I download .epubs on my normal system from either JNC or Nina I get 200 kb/s on a good day and below 10 kb/s on a bad day. Most of the time the downloads fail after a few minutes:

I've tested different OS (Debian, Manjaro, Win10), different browsers (Firefox, Chromium, Konqueror), updated, restarted all systems, routers, switches. I've forced IPv4 only and IPv6 ~~only~~ [EDIT: likely a mistake on my end, an IPv6 only config would not have worked at all at this point.], the issue remains on all systems.
On my phone, if I use data instead WLAN it works like normal. At work I also have no issues. Get up to 4 Mb/s download.
So, at first it seemed to me like a problem with my network but the fact that I have this issue only with these two websites and that others seems to have the same or very similar issues imply that it is not that simple.

I've been trying to figure this issue out for over week now and today I had the most bizarre observations so far. If I hadn't seen it myself I'm not sure I'd believe it.
On the fresh manjaro system mentioned above, I enabled a VPN and the issue is no more (with the VPN enabled):

So far so good. However, if I turn off the VPN and open previously unopened manga with the webviewer the problems are getting "better":

It still takes 5,000-20,000 ms but there are no more "NS_ERROR_NET_PARTIAL_TRANSFER". So in a chapter all pages load until the end and don't stop entirely after a few, making it a matter of patience to read a chapter. I've tested 6 or 7 different Manga and I can't imagine that they have all been cached (again, I did not open them with the VPN active or before that).

I can't explain what I'm seeing and I'm pretty much at my wits' end now. I have no idea what I could do to further investigate this issue. If anyone has ideas or if there is further information I can provide that would be useful, please tell me.
Also if others are seeing similar issues, please let me know.

[Edit: Title changed to solved, strikethrough IPv6 only setup.]

WizardDoc

Website is slow for me also, and app is down/not connecting. I can see the main pages, but no titles will load and open.

shinoubu

I've had this issue for months now too. The website and download speeds are borderline unusable without a vpn. I've already talked about that a while ago on discord in the general chat.

morbelek

Is your ISP provider Telekom? There are reports that Cloudflare protected sites (especially those in the US) are having significant slowness issues when browsed from Germany. Nlookup shows that J-Novel does use Cloudflare.

Eleusis

@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.