Another thing to check would be to use ethtool to see what speed the raspberry pi ethernet port is reaching with your router.

This wasn't on a pi, but it'll look similar. Look at the Speed: line mostly, it'll say either 100MBit or 1000MBit depending on your router (some still only have 100MBit switches). Also look for Duplex: Full, I've had sometimes a config get set badly where it'll be half duplex and cause speed issues, usually due to a bad cable initially causing errors and bad autodetection.

    > % sudo ethtool enp4s0 
    Settings for enp4s0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Supported pause frame use: Symmetric
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 1
        Transceiver: internal
        Auto-negotiation: on
        MDI-X: off (auto)
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes