have you got another device like a relatively new mobile phone you can do a test on (and not an iphone). If you go into the Play store you can download the speedtest.net app and it will automatically locate the closest server and run the test for you. I have got an AC router and an AC tablet (just to confirm that both devices are capable of the speed and this is a true result) and this is an example of what the result of the test looks like from a mobile device. I am on 350mbits so getting nearly 300 over wifi isn't bad going.
Edit: I have just looked up the MacBook Air and if I have got the right model quite interestingly it says
• Connectivity: 802.11ac Wi-Fi networking; IEEE 802.11a/b/g/n compatible
We still need to test the connection on another device to see if it is a MacBook issue and also on a wired connection to see if it is a wireless issue but while you are playing around can you have a look and see if you can get an app for your MacBook called Wifi Analyser. It pulls up all the access points in range, tells you which one you are connected to and what channel etc. I am curious to see how the shub is behaving and whether you are using 2.4ghz or 5ghz. This is mine just so you get an idea of what it looks like:
