@ZER0_o 

There are a few things you gotta look before just reading a speedtest results.  When you log into the controller, goto the client list, and click on the 3 DOTS on the far right.  Then click on all columns.

You will want to look at the channel, Wireless conecction, signal, RX and TX.

Lets make confirm which AP the client is connected to while the speed test is happening.  During that time, lets check the signal levels.  

If the client is not connected to the proper AP (closest one to it) then you will probably enable Fast Roaming.  If that doesnt work, then adjust the Mini Rssi levels on the 5ghz and and 2.4 ghz.  -73 is a good place to start.  

Then you need to look at the signal levels after the change above.  To confirm its getting good signal.

Then you will need to look at the RX/TX if its low numbers, you could have a old wireless client OR there is interference.  Use WiFi Optimization to find a better channel. 

This is the gist of what you need to look at.