I had the same problem except mine was random. sometimes devices would connect at decent speeds other times they would connect at horrible speeds. the meragi go app would show devices connected at 5ghz, but 5ghz is a pretty broad speed range. the properties of the devices network connection would show very poor connection speeds.
there is a microsoft fix in which you go to the driver properties and pick 5ghz as the prefered band....but...since 5ghz is a pretty wide speed range I did not bother with that and that would also mean that each client would have to be modified.
the fix I found by accident was to change the radio settings from "dual band (recommended)" to "dual band with band steering". once I made this change, all my devices that support 802.11 ac connect at 433mbits consistently. I did not see any draw backs for non 802.11 ac devices.