With any VPN, you can expect more latency, unreliability, and slower speeds since you are adding more hops for your connection to travel. The slower speeds come from the VPN having to encrypt and decrypt the traffic going through it since (a normal) VPN encrypts traffic between it and its clients. DigitalOcean, you can normally expect your droplet to have a download speed in the area of 500-800Mbps and an uplink of 250-500Mbps depending how busy your hypervisor or the network in total is. You can check the speed on your droplet using speedtest-cli which is a handy command line tool that uses the speedtest.net servers to test speed. Below is my droplet in NYC3 running speedtest-cli.

To install it, you can run pip install speedtest-cli or apt-get install speedtest-cli. This can help you diagnose the bottleneck you're hitting. Additionally, you want to monitor your resource utilization while running a speedtest while VPN'd to the system. If you are hitting full resource usage, you droplet doesn't have enough oomph to handle your connection.