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The warning is just that; it means Fluent hasn't assigned cpu and has told the OS to sort it out. Typically it happens when Fluent can't identify free cores due to hardware load. We see it on our laptops, and rarely on the cluster if running two jobs on the came box. 

Re the speed. For every partition data must be transferred between the compute nodes. That has a time overhead. As the number of cores increases the time spent on each core calculating the equations decreases (everything gets faster), however the time spent passing data increases (slowing things down). At some point the sheer number of partitions and data passing exceeds the benefit of adding more cores.