Why is synchronization between two devices so slow?

[ScottStanley]

I read FAQ — Syncthing documentation
I don’t use relay
I don’t have any firewall

➜  ufw
zsh: command not found: ufw

Both devices are quite powerful laptops and desktops and both use Linux Mint
Both devices do not use the network for purposes other than synchronization

syncthing v1.27.3

[tomasz86]

What does “so slow” mean specifically? What kind of speed do you get when trying to sync a single large file?