Does the Asuswrt WebUI CPU Graph include AiMesh Nodes? I've notice that when I execute a high cpu and zero bandwidth command on an AiMesh Node that it seems to be reflected within the Asuswrt WebUI CPU Graph.
However keep in mind that if you generate a lot of traffic on a node, that traffic will eventually reach your primary router to get routed to the Internet, which will increase that main router's CPU load.
My initial observation must have been a coincidence. Further high cpu and zero bandwidth load testing on various AiMesh Nodes supports your conclusion.