It's still a problem if you need to do multiple sequential IO requests that depend on each other (example: read index to find a record, then read the actual rec

It's still a problem if you need to do multiple sequential IO requests that depend on each other (example: read index to find a record, then read the actual record) and thus can't be parallelized. These batches of IO sometimes must themselves be sequential and can't be parallelized either, and suddenly this is bottlenecking the total throughput of your system.