Not saying that it is the problem here but often IOPS are the bottleneck. Your NAS may read 1GB/s sequentially but if it filled with HDD in RAID it will likely

Not saying that it is the problem here but often IOPS are the bottleneck. Your NAS may read 1GB/s sequentially but if it filled with HDD in RAID it will likely have terrible IOPS. And if you are accessing many pictures all over the platter that may be a problem.