@xiaobo,
I know that the Ethernet Backhaul functionality of EasyMesh is very new, and may need further optimizations; however I will look into what the actual impact should be, or at least what could be expected.
Looking through the release notes, I do not see wired backhaul compatibility for the AX55, but it may have received it recently. When going to the wired connection, does it properly appear in UI as an EasyMesh extender? It may be causing additional traffic by being in router mode, and needing to be setup as an AP.
It does make sense that the load would be higher when moving to EasyMesh, as the network suddenly can create an actual mesh network with full bandwidth, and this also means that your main router will be in charge of managing all the connections and bandwidth, and moving devices between nodes. However, I wouldn't think it would be anywhere near this much.
When you saw the high load, were you passing a large amount of data through the router? Through the satellite?
How far away is the satellite node(AX10)? Wi-Fi will be subject to the normal falloffs and performance decreases that are seen when repeating a wireless signal.