I use Qnap TS-559 Pro+ and have no issues with speed. The key I have found is to make sure that the NAS has enough ram for access speeds. I have had 2 Qnaps 1. TS-409 PRO and the TS-559 Pro+. The TS-409 is slow when in a raid 5 format while the TS-559 has greatly increased speed when writing and reading.
