For this workload the reporting speeds don’t line up well with the price differences between the RDS instances. I suspect this workload is biased towards R’s CPU consumption when generating PNGs rather than RDS’ performance when returning aggregate results. The RDS instances share the same number of IOPS each which might erase any other performance advantage they could have over one another.
As for the money spent importing the data into RDS I suspect scaling up is more helpful when you have a number of concurrent users rather than a single, large job to execute.
This is an interesting series Mark has going.