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Hi;
I have an Archer C2300 set up as the secondary router (or access point) in my home. The primary router, which is connected to the Internet is a FIOS G1100.
The C2300 is connected to the FIOS router via a Cat5 cable which is plugged into a LAN port on each router. The cable is about 50ft long.

When I run speedtest-cli on my Ubuntu 18.04 box connected to a LAN port on the C2300, I get download and upload speeds > 600 Mib/sec.
But when I run speedtest-cli on my Raspberry pi 3 b+, also connected to a LAN port on the C2300, I get download speeds of <= 100 Mbit/s and upload speeds of up to 260 Mbit/sec.
The fact that I am getting faster upload speed than download speed has me perplexed.
In the Raspberry pi forums, I have been told that I should have Flow Control or Pause Frame turned on for the pi. But there is no such setting on the C2300 (or the FIOS router).

I have tried all sorts of things, including:
Swapping the cables and ports between the fast Linux box and the pi: no difference
Unplugging all other devices from the pi's USB ports (the Ethernet port is on the USB bus on the pi 3), but that made no difference either.
I have run other utilities on the pi, and none of them show a problem with the Ethernet port.

What did make a difference was taking the pi downstairs to the FIOS router, and connecting it to one of its LAN ports. When I did that, I got download and upload speeds of up to 260 Mbit/sec.
This result would lead me to believe that the problem is with the cable running between the FIOS router and the C2300. But I am getting much faster speeds on the PC connected to the C2300, so that seems to rule out the cable.

So I ran iperf to test the local LAN speed between the Linux box and the pi. Here are the results:

iperf -s run on Ubuntu box, iperf -c run on pi
Interval: 0.0-10.0 sec
Transfer: 333 MBytes
Bandwidth: 278 Mbits/sec

iperf -s on pi, iperf -c on Ubuntu
Interval: 0.0-10.0 sec
Transfer: 268 Mbytes
Bandwidth: 225 Mbits/sec

These results indicate that there is a problem between the C2300 and the FIOS router, or with the way one or both of the routers is set up.
I originally had the C2300 set up as an Access Point, and changed it to Router mode to find whether I would have access to Flow Control or Pause Frames, but there was no setting in Router mode. There is no setting for it on the FIOS router either.

Like I said earlier, if it were not for the fast speeds on the Linux box, I would probably be replacing the cable between the routers, but since that is not the case, I really don't want to take the trouble to pull new cable.
I suppose what I could do is bring the C2300 downstairs and install it next to the FIOS router with only a short CAT5 cable connecting the two. If I get higher speeds on the pi with that configuration, then I guess I would pull the new cable.

Any ideas?

Thanks for your help

FW