Bus 005 Device 002: ID 152d:0551 JMicron Technology Corp. / JMicron USA Technology Corp.
Smells like a JMS551 which should be the slower predecessor of JMS561 (USB-to-SATA bridge with 2 port SATA PM). Stange combination since this would mean that's two cascaded SATA PMs inside this Fantec thing. Anyway: it's a USB3 SuperSpeed connection (Port 1: Dev 2, If 0, Class=Mass Storage, Driver=usb-storage, 5000M) but performance sucks obviously. In your case it seems NAS transfers are fast until filesystem buffers (RAM) are full and then contents are flushed to disk.