We're working on making 0 A.D. more parallel, and your outline roughly mirrors our plans. However, multithreading is a lot harder than it sounds - especially with our limited resources. I think the current consensus is that most CPU time consuming part is the simulation (pathfinder in particular) and AI. I few months ago I did a few runs in a profiler and the results are here (see cpuprofile1.svg - cpuprofile11.svg and note that this doesn't show javascript time nicely).