Is it possible that the write load is saturating one of the SS just above the edge and causing it to fall behind?
While high cpu% could indicate that the SS is not getting free cycles to perform the writes, but at 85% cpu, there should be enough remaining for SS to not be starved.
How does the disk io activity look like (iostat -x 2)? It might be good to rule out the disk saturation (use fio to benchmark disk iops) before going to other aspects like read/write hotspots and cpu saturation.