Ok…so I finally solved this by myself and it’s the classic example of how sometimes the most difficult appearing problems actually have a super easy reason/solution. Thus, I even feel a bit ashamed to clarify what was the solution in the end, because it’s just too simple and stupid. I also don’t think it will help many others since it was in the end very setup-specific…but anyways, there we go:
First of all, my setup in short:
ISP --> FritzBox -LAN-> Switch -LAN-> Asus WiFi Router -WLAN-> Laptops, Smartphones, etc.
|||||||||||||||||||||||||-LAN-> Raspberry Pi 3B+
To my defense I have to say that it was actually a combination of two factors making up the perfect illusion that it would be a Nextcloud related problem, which it wasn’t actually.
Problem a) was:
My (headless) Raspberry, although connected via LAN, still had the WiFi actived which is why it was also connected to the WiFi Router at the same time (which wasn’t planned of course). But that explained why the Samba filetransfer was so much faster than the rest: it was using the well working WiFi instead of the LAN connection, which was misconfigured, see -->
Problem b):
Bit by bit I started to narrow the problem down: a speedtest on my Raspberry showed that the internet/LAN connection itself was fine, thus it was not the LAN port/cable/connection in general. The switch was also fine, but it became more and more clear that in the end the problem needed to be the connection between both subnetworks, since the WiFi network itself was also working fine.
In the end it turned out, that deep inside the settings of the Asus Router there was a so called “Quality of Service” option activated, that had a speed limit set to exactly the above mentioned 15Mbit (who does that?). The weird thing is: I can’t recall ever setting this up plus it was supposed to only affect additional guest WiFi networks. Instead, it affected the connection of the two subnetworks between the routers - but not the internet or WiFi connection itself, otherwise I would have obviously noticed earlier. What an unnecessarily long search…and text.
Anyways, now the transfer speeds are on par and I’m more than happy with the result! 