I've had the same issue as the OP on the AX11000 w/386.5_2 firmware and have not been able to find a solution to this. This issue is persistent irrespective of the firmware.
- 10G port from modem connected to 2.5G port on router with Cat7 cable.
- Speed test on Adaptive QoS tab shows 1.3G to 1.5G download and 550 to 600 Mbps upload (modem has its own speed test and test results are as advertised - 1.5Gbs download and 1Gbs upload)
- Oddly enough, after factory reset, with the speed test on the router I do get the expected throughput, but after configuring the router the upload speed is nearly halved. Not sure if it is some configuration or one of the scripts running (Diversion/Skynet etc) impacting the upload.
- 10G port from modem connected to 2.5G port on router with Cat7 cable.
- Speed test on Adaptive QoS tab shows 1.3G to 1.5G download and 550 to 600 Mbps upload (modem has its own speed test and test results are as advertised - 1.5Gbs download and 1Gbs upload)
- Oddly enough, after factory reset, with the speed test on the router I do get the expected throughput, but after configuring the router the upload speed is nearly halved. Not sure if it is some configuration or one of the scripts running (Diversion/Skynet etc) impacting the upload.