The speedup should be measured wrt to the best sequential algorithm on a single node. The aggregate memory on several nodes may allow a different algorithm. Increased cache on several nodes may allow more of the code and instructions to run in core. Speedup is of academic interest. It is possible to obtain perfect speedup curves with a poor parallel algorithm if the single node time is slow.
Speed is what counts.
Part of the job of a scheduler is to maximise the number of computational tasks to run subject to the constraints imposed by the user. This is a non-trivial task. General Tricks Partitioning Divide up the work into a large number of tasks. Agglomeration Gather the tasks together. Mapping Map the tasks onto the processors.