Good question, Corey. I'm not sure about this. I would have thought we had set it a bit lower to not use all your resources.
There is a similar thread on this topic here: https://li360.pitneybowes.com/s/question/0D580000048jc3I/support-for-multiple-processors?
In this specific case, the Concurrency level has no effect on the performance. The Distance Calculator uses only one thread. And this specific case also seems to have the worst case scenario. Finding the nearest distance between a table of polygons to another table of polygons with hundred thousands of nodes.
Typically the Spatial Index of a table speeds up these inspections but with huge polygons, the Spatial Index which typically is based on the MBR of the objects doesn't have a big effect.