I have had this issue since upgrading my internet connection to 2.0Gbps

I had similar problems and in the end, the problem was the switches that did not have the logic and/or buffer to handle the different port speeds. I had to purchase a higher-end switch. There was no problem with the RAX200. Can you please add a drawing of the devices in the mix here and the port speed configured or negotiated speed?.

In my case, I had:

Wireless Device ------RAX200 AP mode --2.5G--Port ---2.5G port  1st GS110EMX--10Gbps port---10Gbps port--2nd GS110EMX ---1Gbps port ----1Gbps port Speed Test Server (internal speed test server)

My download and upload speeds were 300-500 Mbps - If I changed the port connections on the RAX 200 from 2.5Gbps to 1 Gbps my dl/ul speeds were 1 Gbps.

This is when I realized that the switches could not buffer/handle the different speeds between the devices  RAX200 connected using 2.5 Gbps but the Speed Test server on a 1 Gbps port.

Solution: I replaced the GS110EMX with a better/higher-end switch with buffer capabilities and problem solved.

Not all switches have the logic and buffer capabilities to handle these different speeds. I can share the brand and model if you DM me.