Hello,
it seems I have identified the cause:
In my Router, I have limited the bandwidth of all ethernet Ports to 100Mbit/s. After switching this back to 1000MBit/s, the network speed of my LibreELEC machine was stellar again.
As I have also tried going back to 8.2.5 without solving the issue, my current assumption is it has to do with either the NIC or the NIC driver - so in both cases not LibreELEC as a distribution.
Hence, I am marking this topic as solved.