To synchronize 2 sites which are geographically distributed Syncthing has to pass through many hops. Single TCP (or any other) connection which is used to transfer data is very limited in bandwidth by magistral or local providers.
It usually happens because traffic exchange points are heavily loaded, traffic shaped, or networks misconfiguration etc.
It is easy to check via speedtest.net by selection of single/multi mode and some server in another country (or even continent).

For example, connecting from Europe to USA in single connection:

And the same setting, except multi-connections:

In my specific case situation even much worse, it results that on 200 MBit/s connection Syncthing exchanges data in peak hours with speed 100 KB/s, and up to 2 MB/s at night. In comparison aria2c with 100 connections even in worst cases can utilize full bandwidth and downloads file with speed 22-24 MB/s.
Increasing of speed is dramatic... Even 2 MB/s is very slow but 100 KB/s makes Syncthing unusable for scenarios with geo-distributed sites and a lot of hops.

So it is crucial thing for Syncthing to support multi-connections like it is made for example in aria2c.