I'm using OMV with proxmox kernel and this dkms:
https://github.com/awesometic/realtek-r8125-dkms/
I have the same slow speed problem for some times.
I have a r8125.conf file in /modprobe.d with:
options r8125 aspm=1
options r8125 eee=0
I changed the last line to
options r8125 eee_enable=0
rebooted, now full speed is back!