After lots of tests - here is the problem completely reproducible using iperf3 to create 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/upload to WAN at high speed if there is LAN traffic at the same time.
I have to add 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.
@sppmasterspp what we should really test is if the original firmware also suffer from the same problem. Can you that simple test? It's just to have a direction if internally they fixed the bug and we are just missing the workaround on our code
@Ansuel What I've been able to test so far is this:
With Voxel firmware based on original Netgear firmware the router can reach maximum download speeds (considering the scenario with LAN traffic and 100Mbps Problem client takes place) even with ping being 50-70 ms. But the WAN upload speed in this case is limited to around 150Mbps.
Will add more test data as soon as I can.
I've tested extensively the latest stock Netgear firmware and latest Voxel firmware for R7800.
I've found that both have almost the same issue as OpenWRT.
There are a few differences. The stock firmware is more resilient and when the LAN traffic is present (between PC and 100Mbps client) the full WAN download speed is still achievable even though with higher ping times. But when the WAN speed is at full there are drops in LAN transfer speed.
The upload WAN speed is limited as with OpenWRT firmware.
Original Netgear firmware experiences issues with WAN download speeds too but this happens when there are two different 100Mbps devices connected to the LAN ports and there is a LAN transfer involving one desktop PC (1Gbps) sending files over LAN (or running iperf3 server) to both 100Mbps clients. Using this scenario the R7800 has problems to achieve full WAN down/up speeds.
If the clients are connected at 1Gbps full WAN/LAN speeds at low pings are possible.