I have a similarly provisioned device from Kingston and see about the same performance. Your MAC CPU is probably 4-6 times as fast, multiple cores and AES (hardware encryption) as well as many times the RAM as the GL-MT300A. OpenVPN is very resource intensive. These speeds should support most streaming services with little issue.
You have not stated your use case (Travel, home, etc) but I find that if I can have at least one side connected by Ethernet I can pick up a few Mbps. I assume it’s taking some of the load off the processing for the radio.
I understand that the router has limited CPU and it is already doing pretty well. But, to be honest, if I knew this limitation probably I wouldn’t buy it. It’s a nice piece of hw, I love it, but it doesn’t fit my needs. Don’t you think that it would be better to specify clearly what is the max speed with OpenVPN turned on?
Before returning the router to Amazon: is it possible to turn off VPN encryption? I already edited the ovpn file provided by Astrill adding “cipher none” but then it can’t connect anymore.