I've tried several things today and definitely this looks like a software bug.
I've used three routers - two different R7800s (to reject it is a hardware problem only) and cheap no-name gigabit router with Ralink chipset.
My previous tests were only with a Desktop PC, two Android TV set-top boxes and a Smart TV set. Today I've added one Laptop connected by cable via gigabit NIC and second Laptop connected wirelessly via MediaTek MT7921 2x2 802.11ax (WiFi 6) Card .
I've made the same tests consequently with all three routers.
I can now confirm that the two R7800 routers experience absolutely the same slowdowns/drops of WAN download/upload throughput in case of even moderate LAN traffic.
After lots of tests here is the problem completely reproducible using iperf3 to make LAN traffic between LAN clients. In my case I run iperf3 (with a predefined throughput of 60Mbps) between a Laptop connected at 100Mbps! (this is the culprit) and PC connected at 1000Mbps. The result - the PC can only download/upload from/to WAN at really low speeds.
If the same test is performed when a Laptop is connected at 1000Mbps then there is no problem for the PC to achieve full speeds.
The slowdown only occurs when a device (Laptop, PC, Smart TV) is connected via cable and the negotiated speed is 100Mbps (I've used a 100Mbps cable). Then the other device even connected at 1000Mbps cannot download at high speed.
It's really tricky when there is an additional switch (even a gigabit one) connected to the router because I can see the link speed is 1000Mbps, but some clients have only 100Mbps NICs and when there is LAN traffic between the clients this becomes a nightmare because all the clients cannot download from Internet. The speed sometimes is just 5-6Mbps.
This definitely should be fixed because otherwise the Internet connection is almost unusable when we have LAN traffic. At least there are no problems with WLAN clients speed.
No similar slowdowns are observed with the third cheap router I've tested.
I have to stress that two compulsory conditions should be met at the same time in order to reproduce the WAN performance drop. A client connected at 100Mbps (let's call it "Problem Client") and a LAN traffic between the "Problem Client" and any other device connected to LAN by cable.
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