The ethernet adapter can make a difference as well as the version of the network driver it's using. Other components in the PC can make a huge difference also. I have a gigabit LAN at home. Backups from one PC to the other used to take 5 hours or so. I replaced the drive that it backs up to and the backup time dropped to one hour. This is without changing LAN adapters or their drivers. Both drives were in good working order. The only difference is the slower drive was 5 years old. Basically new systems have faster buses, faster drives, and faster ram.
For a simple test, have you tried switching the ports the PC's are connected to at the router? This will at least tell you if both router ports are functioning properly.
Yes i did. I switched the ports around, switched the cables around too.