"speedtest between LAN and LAN is nonsense"

Speedtest would indeed bring wire-speed, that is the full ethernet speed, and would be better than a WAN-LAN connection. I do not expect only 1/3 of the speed in WAN-LAN.
WAN-LAN is faster in bridge mode than in router mode, but still that WAN-LAN bridged PC connection is expected to be slower than in LAN-LAN switch connection in the SFT1200/OPAL.

TCP congestion problems ... "Usually happens when different network types are mixed" I mix which different networks ?"

It is not clear to me if you use wireless (or VPN) somewhere in the test setup. If not then only internet provider network and your ethernet are involved. Thats only 2 types of network then. (Speedtest. net is over the internet connection normally)

GL.inet does not provide us with a block diagram AFAIK. So let me try to invent one.

EDIT: some more information from SiFlower hardware chip. Explains WAN-Wifi and LAN-Wifi , but not specific WAN-LAN information. 以太网到WiFi加速流程介绍 — 矽昌通信
"https: //siflower.github.io/2022/03/11/hnat_snat_acceleration_guide/"

More info should be here (a lot of information !) , but GMAC is common to WAN-LAN bridge traffic and WAN-LAN router traffic. 以太网收发包接口说明手册 — 矽昌通信
"https: //siflower.github.io/2022/03/11/ethernet_trx_guide/"

There are many other documents in that wiki. Like 以太网测试介绍 — 矽昌通信 "https: //siflower.github.io/2020/09/08/ethernetTestGuide/"

And more ? : Siflower路由性能测试手册 — 矽昌通信
"https: //siflower.github.io/2020/11/30/siflower_throughput_guide/"