Does 0ad use multi cpu cores, or just one?

When you mention render submission being the main bottleneck, is the core issue that Pyrogenesis's rendering pipeline runs sequentially on a single CPU thread, or is it already multithreaded and just struggling with massive draw call volume late-game? Furthermore, is there any prospect for optimization through GPU instancing to batch identical units and cut down those draw calls?

Is there another way you can use hardware when not all of it is being used, so it can free of some when you need it more?

Pathfinding is not actually as performance expensive as one would think. The individual pathfinding requests each turn can be (and are) nicely computed in parallel by the engine, so on machines with decent hardware concurrency they finish surprisingly fast. The render submission process and rendering itself are a much more perfomance-constraining factor.

Indeed, and unfortunately you cannot pre-calculate all possible paths. That could be possibly an acceleration factor.