Honestly, the answer is yes, looks fairly normal, but I'd consider replacing anyway.
Speed is probably the third criteria I'd use for considering which drive to upgrade. First would be SMART status, second, raw capacity, third, speed.
If you don't need more space, don't bother replacing anything until it shows signs of failure.
However, if you want to pro-actively replace stuff, get another 8TB drive, replace the slowest 2TB outright with a rebuild, copy the contents of the other slow 2TB drives to it, and remove them. File system type plays into this as well, if the 2TB drives are ReiserFS the strategy changes.