Sync v2.5 introduced option to optimise speed of syncing across the internet - WAN optimisation. This option is available with Sync Business license.
Sync uses new Micro Transport Protocol (uTP2) to intelligently transmit data across the network. This protocol can detect available bandwidth or network congestions and adjust its behavior accordingly in a smart way - increase data transit if possible, or avoid data loss and unwanted packet retransmission on a busy pipe.
To start using the optimised data transfer:
1. Install Resilio Sync Business (verify in Sync UI header that it's not Home!).
2. Purchase the license with the necessary number of WAN seats. 
3. Apply the license and see the purchased options appear in License menu:
Share the license seats with other Sync installations.
If running Sync in config mode, add parameter "net.enable_utp2": true to the config file and restart Sync. These devices must have license with WAN option enabled.