As Rafael says, test with various core counts and work unit counts.
And "fast" here can be described in two ways. One is throughput, or how many units you can do in a set amount of time. Two is fastest unit. They are not always the same. So it is best to test to strike the balance however you want it.
Also, there is no limit of how many cores you can throw at a single task. The more cores the faster the task, but you might get more tasks done by running more than one task at a time within the same time span.
Badge Score: 4*2 + 5*3 + 6*10 + 9*2 + 10*1 = 111