I have not used PVE 9 yet. What kernel version does it use?
There is this bug in the bug tracker, specific for kernel versions 6.8.12, but I have not been able to find a more recent one for PVE 9.
This is a problem with Intel cards.
I have several servers on Hetzner, all of them with Intel cards, had the same problem..
So, i followed their advice..
https://wiki.hetzner.de/index.php/Low_performance_with_Intel_i218/i219_NIC/en
with this...
For future reference, as I presume users will keep on visiting this thread:
dpkg -l |grep proxmox-kernel
proxmox-boot-tool kernel pin 6.8.12-8-pve
And reboot.
P.S. Not sure if I already posted this before... :D