So you had a working setup with VPN speeds around 50MBit/s and without touching anything it's down to 10MBit/s?
Definitely sounds upstream/ISP to me. Not implicitly incident, maybe they capped some Ports or low QoS VPN traffic?
You are in TLS Authentication mode? Try TLS Encryption and Authentication.
Also try to change Port for OpenVPN, e.g. some real-time application port like SSH (22) for testing.
Should not be your actual problem but I'd go for AES-128-GCM or AES-256-GCM not CBC.
-Rico