Hi, problem can be with MTU or TCP data window size. I can troubleshoot it and fix.
I am managing more than 1000 ovpns, most of them on Mikrotik routers but also some on Debian/Ubuntu.
Anyway problem is not with software but most problably theoretical, how networks work and tunnels.
I will set up some parameters and it will run fine.
I am network engineer ;) I can help with it right now.