Volume operations causing slowness across SMB

As others have stated, WD Reds are about the slowest NAS drives around. Nothing wrong with them, and they're priced accordingly, but they're relatively slow. We use WD SE drives in our 516.

Given the slowness you report seems to involve long sequential writes (e.g. saving a 2GB file) - even with the Red drives, with a ReadyNAS 516, your first limitation is almost certainly a network bottleneck if you're only using a single NIC on the 516. You need to get a managed switch that can do LACP and bond both gigabit nics on the 516. That way, when one of your (gigabit, I'm asssuming) network clients is saving a huge file and saturating one of the gigabit links (only 100MB/sec) there's at least some hope for some file i/o to still happen over the second link. But even bonded gigabit nics will net you 200MB/sec theoretical peak, which isn't terribly fast if you're doing big file i/o.

If large file manipulation is the name of the game and you have the resources, you'll probably want a 528X, use 7x00RPM drives like the Gold (or SE, or Red Pro), set up them up with RAID-50 and connect it a switch with 10 gigabit capabilities. Maybe even hook up some of the heavy user clients with 10 gigabit nics too.