The OS is in a spanning RAID1 across all internal disks, RAID1 speeds are bound to the slowest disk in the array.
If you add SSD’s you need to start from scratch as you have two system volumes now fighting. (so kill and restore from backups)
As the OS is on all disks, all disk would constantly spin as any OS read or write keeps all disks awake.
Just put the NAS out of earshot and it should be fine.