Do you have rate limiting enabled by any chance?
It seems that one of the devices pretty much can’t talk to the other one, but that’s due to some network/OS problem, not syncthing.
I use syncthing to have my photos on all these nodes. Sync between A and B works okay but when I start the nuc the laptop shows 75%(phone), nuc shows 50%(phone) and my phone shows 100% for the folder.
And check CPU usage using top on the Pi. I’ve seen reports of slow devices churning “something” for minutes after connected before starting any activity…
So I checked CPU usage using top on the pi while the syncing is stuck at 0%. There’s no significant usage. I’ve never used iperf (don’t know anything about it), so I’ll be a bit longer before I can return with any info based on this.