If you are setting a static 5ghz (dual-band) channel on your wireless router, that may be the problem. Try setting your router to "automatic" if you can.
This can be accomplished by taking the following steps:
- Open up spotlight (cmd+space) and type "airport utility" if you are running an AirPort router of any type.
- Click on the icon for your airport, then click on "Edit"
- Click on the "Wireless" tab
- Click on the "Wireless Options" button
- change 5GHz channel to "Automatic" from the dropdown menu.
- click save
This will restart your AirPort and should fix your Retina Mini wifi problem. I am getting 100Mbps+ down and 100Mbps+ up on gigabit AC wireless.
As to why this is happening? No idea, and it would be a waste of time to even guess. Cheers!
P.S. If you have a non-airport router, you can see if this problem translates by logging into your router's admin panel. Steps for access should be in the instruction manual that came with it.