There seems to be a huge difference between spin-down while NAS is up vs shutting the whole NAS down and restart. When I start my NAS, it takes a bunch of time to be back up: it seems to do a lot of checking on/ syncing off the drives and puts a fair amount of load on them (same is true for CPU as well, just look at your CPU load right after startup).

OTOH, when the NAS spins up a single disk again, I haven't notice any additional load. Presumably, the read operation just waits until the disk is ready.