I have a GL WT300N-v2 router. Without VPN I reliably get 70Mbps download speed on its WIFI (on 95Mbps link). With OpenVPN client (for two VPN providers - TorGuard and ExpressVPN) I never get more than 6Mbps. With the same VPN settings on my Windows computer I’m getting much higher speed (close to saturation of my provider link - 80-90Mbps).
I’ve got the latest Firmware, and I haven’t tweaked any settings.
What’s your speed with VPN connected (OpenVPN client)?