A number of factors can affect how quickly your files transfer between devices. Tuning up one or several things mentioned below may help you increase the speed of synchronisation.
1. Direct connection.
One of the most significant factors is whether your other devices are connected directly (i.e. appear on the same LAN), or indirectly (relayed).
Generally speaking, "direct" connections are preferable as they are significantly faster than relayed connections. Therefore, the first step to improving sync speed would be to try and ensure that direct connections are established between your devices. If they all reside on the same physical local LAN, this shouldn't be a problem. However, if they are "remote", you could consider establishing a VPN in order to make your remote devices appear as though they're all on the same local LAN.
2. Syncing in LAN (local network).
Once your devices all appear on the same LAN, follow the steps in the article "Can I force Sync to do local network (LAN) syncing only and not sync via the Internet?" to restrict syncing to your local LAN.
Next, if possible, assign static IPs to your devices, and then use the "pre-defined hosts" settings (again, these are per folder) to list your devices. At the same time, untick the "Search LAN" option. By doing this, Sync will then know exactly where to connect without having to spend time searching for other devices.
3. Speed / Peer limits.
Make sure you have not set any receiving/sending rate limits in Sync general preferences, and the Advanced - power user, "rate_limit_local_peers" setting is set to "false".
4. Data encryption.
Encryption of the data you transfer takes a lot of resources and hampers the synchronisation speed. Set "lan_encrypt_data" setting to "false" in Advanced preferences - More options.. to potentially further improve transfer speed.
5. Resource usage.
If you have a more powerful device, you could also consider setting "disk_low_priority" to "false", and increasing the size of "recv_buf_size" and "send_buf_size" to get a further speed advantage.
6. Disk fragmentation.
One final thing that can improve overall performance of Sync is how fragmented your hard discs are. The less fragmented, the better indexing speed will be - so if you've not de-fragmented your drives for a while, it's worth doing!
If you've tried the tips above but syncing is still pretty slow, see other factors which may affect transfer speed.