Tranferrates while copying Sata to Sata

Usually a you should get much higher speeds, but when copying whole drives caches are not very useful. So you may experience much slower speeds than what SATA III is capable of.

Cheap hdds may rely more on the caches for speed. And they are fast only in short bursts. Better, more expensive drives may have faster spinning platters and faster internal electronics and will have higher sustained throughput. When disks are reviewed this is often tested.

Also when disks start to fill up they slow down. That may depend on the filesystem as well as the fact that more inner tracks are used, not the better outer tracks.

It is also possible that you get thermal throttling during long tasks. Especially if you use encryption or compression.

And if you use a foreign filesystem (NTFS) then that may also slow down the transfer.

Also rsync is probably not the fastest way to copy data internally. It does more than just shuffle data...

No, IronWolf is a pretty good HDD. Not the best but far from the worst. I use IronWolves 12TB myself.

Still IronWolves can not do sustained copies at full SATA III speeds. I don't know if any SATA III HDD (or SSD) exists that can do a sustained full drive copy at full SATA III speeds.

Less is more...

I haven't compared speeds, but I assume that:

cp -a source destination

...is hard to beat for a local copy. It does the same as a similar rsync command. Recursive copy and keep attributes. Good for testing speed or restoring a backup but rsync is probably better for making backups...