In my effort to move away from Dropbox, I was testing Synology CloudStation with my own Synology NAS at home, I was very frustrated with the extremely slow speed.
Because I was still in testing mode, I used a Synology feature called “Quickconnect”, that I thought would work like a Dynamic DNS. While it usually does, if you use it together with CloudStation, it seams that the traffic is routed through Synology’s servers, which a) makes it incredibly slow and b) kind of defeats the purpose of more security. (The traffic is still encrypted, if you use SSL)
The solution to the problem is very simple: Do not use Quickconnect and connect directly to the DiskStation. If it has a public IP address open TCP port 6690 or forward that port from your router or firewall if the DiskStation has only a private IP address, e.g. in your home network. You will get a much better performance.