That would equate to around 120MB/s read and 59MB/s write speed, so the connection is not the issue.
The next step would be opening
top and then transferring a big file, watching the overall CPU load and which processes are the highest.
Edit: With yesterday's snapshot build (r10586) and currently latest cifsd (kmod-fs-cifsd 4.19.57+2019-07-17-0c3049e8-1) I get the same ~42MB/s read and ~35MB/s write I got before, maxing out the CPU in the process. This is on a WD My Book Live which, hardware-wise, shouldn't be inferior to your Zyxel NAS.