Hello, i have Zyxel 326 Nas, it has two internal 3.5HDD and two external 2.5HDD. I´m only able to transfer at maximum 25MB/s to them. I can download from internet at 60MB/s. I have Xiaomi AX3600 router and i´m transfering to the Nas from WIFI 6 (AX3600 supports WIFI6).
Two 2.5" or 3.5" SATA II hard disk interface supported up to 32 TB (16 TB x2)
One Gigabit Ethernet RJ-45 connector
Two USB 3.0 ports
One USB 2.0 port
If it is correct that it is using SATA II ( and not a typo), that might explain some of it. Also if you are transferring to/from one of the USB attached disks, that is also likely to be slow.
The numbers they market with will be for sequential read/write which is the fastest. Random read write will be much much slower. Also, the raid type can make a difference, but not usually as much as the difference between sequential and random R/W.
Use Iperf 3 to test the transfer rates directly between a PC and the NAS connected by a single direct ethernet cable to eliminate the router and wired or wireless connections.
What type of transfer are you doing - folder copy ? single large file copy ?
Typical folder copy using SMB protocol can easily be in the 15-30 MB/sec range. A single large file may read or write much faster. Look at the NASPT test results for this NAS on the site or similar ( same processor and drive interface) for comparison. i think you can still download the NASPT test program from Intel, but i haven't looked in years.