Model: GL-AR750S-Ext
GL-Inet Software: 3.104
openvpn-openssl: 2.4.5-3
WAN connection 250/250 mbit/s
I think I have the same or a related problem with the same setup as you.
With OpenVPN client connected to a pfsense box and VPN Policy enabled to only route traffic designated for the remote LAN through the tunnel I get Internet speeds around 1.5/0.5 mbits.
With OpenVPN Client disconnected speeds are 250/250. By looking at the routes in the AR750 and using tracert commands I can see that all traffic except the one designated for the remote LAN is not sent through the tunnel.
Can’t really understand why non-VPN traffic get so slow.
EDIT: Look here and try the firmware in the link further down in that thread. It worked for me