Spin up all raid drives at the same time
The drive does a staggered spinup likely because you access sequential data chunks. Suppose having a stripe size of 64K (single disk chunk size of 16K): a single 64K read will sequentially wake all disks.
To avoid spinning down the disks you can use hdparm -B 128 to selectively disable spin down only. For maximum performance you can issue hdparm -B 255, which completely disables any power-saving measures.