Ethernet Performance Problem

[Marc Auslander <marcslists@gmail.com>]

I have an Realtek Semiconductor Co., Ltd. RTL8101/2/6E PCI Express Fast/Gigabit Ethernet controller (rev 02) Subsystem: Acer Incorporated [ALI] RTL810xE PCI Express Fast Ethernet controller

There is also a Realtek Semiconductor Co., Ltd. Device 8161 (rev 15)
    Subsystem: Realtek Semiconductor Co., Ltd. Device 8168 100BaseT not being used.

lspci -v says the driver is R8169 for both.

firmware-realtek is installed and does not appear to provide R8169 but I'm a novice about these things.

    The cable leading to the debian computer, when connected to a
    different computer, runs at almost 1000 Mb according to iperf3.
    
    When talking to Debian Buster it runs about 100Mb give or take.
    
    ethtool says its running Speed: 1000Mb/s Duplex: Full
    
    I just noticed this - in the past it ran at 1000Mb/s rates. It may
    have happened when I recently went from squeeze to buster, but I
    can't be sure of that.
    
    Any suggestion on how to proceed.