So imagine that your queue looks like this: SSSSSFFFFF Now you will always be struck processing the slow S tasks before getting to fast F tasks. All the work

So imagine that your queue looks like this: SSSSSFFFFF Now you will always be struck processing the slow S tasks before getting to fast F tasks. All the work has to be done, but the F tasks will always have to wait a long time. The queue would just rotate doing this. This is in contrast to random dispatch which will result in roughly this: SFSFSFSFSF Now you get to execute more tasks per minute despite not really increasing your throughout capability.