Slow book downloads

[hasezoey]

I have recently been experiencing slow book downloads, i had also written previously in discord but then it had resolved itself.
Slow as in 150kbs or less, where for reference normal book downloads are in range of 7 to 10mbit.
I am opening this thread because this problem has now persistent for ~3-4 months, mostly affecting firefox.
As a workaround i have been using wget (wget --content-disposition URL) which mostly works-around the issue, but even there it still sometimes gets very slow (even to the point of 0kbps, but still not timing-out).

From my testing it can be fixed by stopping the slow wget and restarting, and if that does not fix it, try again until it is, as for firefox it mostly stays at the 150kbps, even after restarting and only rarely has the full speed nowadays, though once it has been downloaded full, and re-downloads (for testing) are full speed, even in firefox.
This also seemingly does not affect every book equally.

Firefox 121.0 (but also had lower versions in those months)
wget 1.21.4

recent examples i tried to download but were really slow:

• 8th-loop-for-the-win-with-seven-lives-worth-of-xp-and-the-third-princess-s-appraisal-skill-my-behemoth-and-i-are-unstoppable-volume-1.epub
• how-not-to-summon-a-demon-lord-volume-3.epub
• i-shall-survive-using-potions-volume-8.epub

PS: after writing this, every book i tried to download (via firefox, for testing) has been full-speed, so maybe it somehow is only related to (very)-recently bought books (like buying and directly downloading)?