Sorry but I have to ask the obvious, the speedtest node is in Frankfurt, is your node also in frankfurt?
Also, keep in mind that speedtest nodes are going to be mega-optimized for ping, traceroute, bandwidth, and latency. So they’ll likely be sitting on the edge of the network not behind X routers, and Y distribution switches along the route. That’s because the speedtest results from these nodes are going to make or break a potential customer.
Another possible factor is the firewall layering. Are you using DO’s firewall as well as your node’s firewall? Iptables rule processing will likely add to latency.