@Talkabout
So i tested OpenVPN (installed by using PiVPN script) and got following results. Phone was connected with Wifi to the same local network as RockPi and RasPi. Both of them connected by cable
No VPN (Download/Upload Mbps)
42.8 / 41.8
RasPI PiVPN
31.1/30.0
RockPI PiVPN
37.1/39.1
So from this i can conclude that problem in your network