There is actually another command which can show the process itself and what happens with the read/write etc.
sudo fs_usage syncthing
which in my case has an output like:
16:32:36.098645 write F=32 B=0x1000 0.000008 syncthing.1030685
16:32:36.098658 write F=32 B=0xc97 0.000007 syncthing.1030685
16:32:36.098667 read F=32 [ 35] 0.000001 syncthing.1030685
16:32:36.138932 THROTTLED 0.715509 syncthing.1030732
16:32:36.138933 read F=25 B=0x8000 0.872152 syncthing.1030732
16:32:36.224242 THROTTLED 0.794747 syncthing.1030714
Any THROTTLED as far as I know means that the MacOS is throttling resource heavy processes for idle applications. Why the Syncthing is considered as an idle application I have no clue, the developers can probably answer and fix this.
How to temporary fix it?
Prior folder scan you can run:
sudo sysctl debug.lowpri_throttle_enabled=0
To enable the throttling again you run:
sudo sysctl debug.lowpri_throttle_enabled=1
You can as well run
sudo sh -c 'echo debug.lowpri_throttle-enabled=0 >> /etc/sysctl.conf'
to disable the throttling on your system completely (no clue what will be the long term impact).
With disabled throttling the scanning performance is in my case ~270MiB/s (the value 8 for Hashers seems to be good for my storage).
Thank you very much everyone for your help!