Hi, I had a similar problem that was produced by my network card, this network card had problems with the checksum offloading.
Try the following commands:
Check the current configuration:
# ethtool --show-offload eth0
Temporarily disable the offloading:
# ethtool -K eth0 tx off tso off gso off
check the changes:
# ethtool --show-offload eth0
Repeat all steps for each network interface
Check the upload rate
If the problem is solved you must make this configuration permanent by modifying the network module script in the hooks folder, a more detailed description can be found in:
https://guidocutipa.blog.bo/mejorar-rendimiento-red-zentyal-proxmox/