Solved:
This turned out to be an array level issue related to VAAI. We are running R31 code on our VNX storage array and 'VAAI nested clone support' didn't come into play until R32/INYO for VNX. I have temporarily switched to FC storage since doing an upgrade right now is not an option for us, but I did some testing by mounting some NFS datastores from an INYO array to these hosts and the same tasks succeeded without issue (very quickly I might add).