WRT1900ACS: low NFS speed

I've re-activated a WRT1900ACS with OpenWrt 19.07.2 r10947-65030d81f3.

I've connected to the USB3 port an USB3 memory stick and get a transfer speed with dd if=/dev/sda of=/dev/null bs=1M count=1024 of about 55 MiByte/sec.

When I mount this stick via NFS from a client PC, I get a transfer speed of about 1 MiByte/sec. I think that's to low.

I have a GL.inet MT300A with an internal sdcard reader. Reading the sdcard internally with dd if=/dev/mmcblk0 of=/dev/null bs=1M count=1024 I get a transfer speed of about 23 MByte/sec and via NFS of about 12 MByte/sec.

The mount options on the client PC are the same for both routers.

Any idea where I should look and investigate?