Syncthing only uses one TCP connection to transfer data. This unfortunately means that if your ISP/network limits the speed intentionally (or on accident) per TCP connection you are limited to the single TCP connection speed.

One reason BTSync is so fast is that it uses multiple connections (even between the same two servers).

In my case, my ISP doesn't intentionally limit the single TCP connection speed, but in practice it is limited between 5 and 8 Mbit. Whereas if I open up a torrent client (yes, I know that is torrent protocol, not Block Exchange), I can max out the full 50 Mbit I pay for.

I have tested that it is not a bug in the download speed of syncthing, by intentionally leaving one node offline (out of sync), and then letting all of my other nodes sync up, then starting up the one node. Then I get 5X (number of nodes) Mbit instead of just 5.

Before you say, upload bandwidth is the problem on the server I am downloading from, it isn't. The aforementioned cluster of nodes is on a 100Mbit Symmetric connection.