You don't mention what the Ethernet cable is plugged into on the System end. Also, you have checked or changed out your Ethernet cable?
If you were to have a Dell system, there may be something named SmartByte that could throttle speeds.
When something starts really fast, then slows down, it might be some type of buffer is being filled and once it is, the speed slows.
What does the Link speed show for your Ethernet network?
Knew I probably missed something...
Yes, I've tried different Ethernet cables between the Arris S33 modem and the GT-AXE16000 (both CAT6 and CAT8) to see if the cable was a problem. Same symptoms persisted.
I have been testing my system for the last couple of hours and am not that happy with it either. It may be the file I am transferring, but I should be able to get at least a consistent 1 Gbs but on some transfers, I am not getting much more than my ISP speed, which is around 600 Mbps.
More research will be done, even switching back to an AXE11000, may be necessary. I will also be ordering new networking cables.
I was going to ask if you guys have enabled Jumbo Frame under Advanced Settings LAN -> Switch Control?