It will be good to have possibility for automatically set order of execution of matrix subtasks based on tracked time(in execution history) - longer subtask must be executed earlier then shorter. I think, it could help to decrease execution time for full matrix task.
F.e. I have 2 nodes and 3 matrix configurations(1,2,3) and estimation time of execution based on history is 1h ,2h, 3h. I suggest to write plugin which set execution order: 3->2->1. In this case, full execution time will be 3h (node 1 - task 3, node 2 - task1+task2). In any other order full execution time will be longer.