2015 MBP with Catalina - slow Thunderbolt to Gigabit Ethernet upload speed

[donnieput]

Here's the environment. AT&T 1G fiber & modem, new CAT 6A cable, new Apple Thunderbolt to Gigabit Ethernet adapter, two different MacBook Pros. Three test cases:

1. fiber modem to cable to adapter to Thunderbolt port on 2011 MacBook Pro El Capitan
2. fiber modem to cable to Ethernet port on 2011 MBP El Cap
3. fiber modem to cable to adapter to Thunderbolt port on 2015 MacBook Pro Catalina

Same speed test software and server

Here are the results:
Test cases 1 & 2: 950 Mbps download and 950 upload
Test case 3: 950 Mbps download and 700 upload

Any explanations of why the upload speed is slower and suggestions on how to remedy it?

Thanks much! Don