Its a system, not an IO, bottleneck typically. Especially with RAID 6. Its math that runs on a single thread.
Distributed storage systems with per object raid FTW here. If I have every VMDK running it's own rebuild process (vSAN) or every individual LUN/CPG (how Compellent or 3PAR do it) then a given drive failing is a giant party across all of the drives in the cluster/system. (Also how the fancy erasure code array systems run this).
Yeah, that's RAIN and that basically solves everything 🙂