To all owners of Belkin RT3200/Linksys E8450. Attention to @quarky who confirmed the behaviour and results and @Ansuel.
I own Netgear R7800 and I've reported a bug here https://github.com/openwrt/openwrt/issues/9202
R7800 poor download rate when client is connected at 100Mbps.
The exact steps to reproduce the bug are here.
Netgear R7800 exploration (IPQ8065, QCA9984) - #3187 by sppmaster
Today I've benchmarked a Belkin RT3200 with the same tests with a 100Mbps client connected to the LAN switch port of RT3200. It still runs factory firmware. I can hardly reach 200Mbps down and 20Mbps upload speeds.
I can only say that I see the same decrease in the WAN throughput. The same issue is present as with R7800. So probably the bug is not connected to the specific hardware as both are based on different SoC.
Probably the reason is the same regarding this issue too https://github.com/openwrt/openwrt/issues/9420
Is it clear what exactly cause this erratic behaviour.