Ethernet Performance Problem

[Marc Auslander <marcausl@gmail.com>]

I have an Realtek Semiconductor Co., Ltd. RTL8101/2/6E PCI Express 
Fast/Gigabit Ethernet controller (rev 02)

lsmod says the driver is Realtek. firmware-realtek is installed

The cable leading to it, when connected to a different computer, runs at 
almost 1000 Mb according to iperf3.

When taking 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.