I have the same problem on the GL-AR750S-Ext router. I’m running the latest firmware (3.104) and the issue still isn’t fixed.
I’m using OpenVPN with TorGuard. When I set the Policy to only allow VPN on certain MAC addresses, the internet on devices which aren’t in the list become so slow that they aren’t even usable. Often times the internet speeds on non-VPN devices will drop down to 0mbps while the VPN devices continue working just fine with normal speeds. It seems like something seriously bad is going on, almost as though the router is having trouble distributing the traffic properly. Maybe a cache setting somewhere? This doesn’t appear to be an issue on the MT300N-V2 Mango. Could it be a hardware issue? Bad firmware? What’s the deal here. Other’s seem to have the same problem and the issue still hasn’t been resolved.