If you are encountering very slow download/upload speeds, check the following reasons why it may be happening:
- Transferring a big number of small files. Download/Upload Speed will be slower for many small files as when transferring a few large files.
- You are using Relay Server. Your peers cannot connect directly. Check your firewall properties (it should not block incoming packets to Sync's listening port) and your NAT settings. Direct port mapping might help.
- Asymmetrical peers connection. If there is a peer with a slow uploading speed, other peers will have low downloading speeds. It also means that the more peers with high uploading speeds, the higher the downloading speed will be.
- Low capacity modems. Network hardware that cannot process big flows of data packets at a time will affect the speed of Sync.
- Firewalls and/or security software settings Network security and Anti-virus services may block/delay data transfer or the re-checking of the files as they are downloaded. Try temporarily disabling these services.
- ‘disk_low_priority” is set to "True" in Advanced settings.Try changing this setting to "False", option available on desktops only.
- Sync's Listening port is not open. To open this port, make sure "UPnP port mapping" is enabled on both ends, and in router, if the network/router supports UPnP.
- Try port forwarding via router interface (TCP/UDP). The list of ports used by Sync is here. Consult your router documentation for instructions on how to open ports in your network.
- Try using predefined hosts. You can find predefined hosts in the folder preferences.
- ISP may be throttling your traffic.
For additional tips on sync speed improvement, refer to this article.