The DNS-345 is slow because of it's slow processor. Test it yourself:
Log into the webinterface and open Management > System Status > Resource Monitor
Now copy at least 1 GB to the NAS and check the CPU Utilitazion.
After the file is copied to the NAS, copy it back to your Computer and watch the CPU Utilitazion again.
The bottleneck is the CPU. HDDs and LAN is fast enough! That's why i also bought a Qnap TS-459 Pro II - it costs more than twice but has a much faster CPU, upgradable RAM and more professional features like iSCSI and such things.
With the same HDDs and settings (4HDDs RAID0, no encryption, etc.) i get ~35MB/s with the DNS-345 but ~120MB/S with the TS-459.