Problem Wifi Low 5Ghz

Hello, I don’t know how to solve this problem, I have a Mikrotik RouterBOARD 952Ui-5ac2nD that works with two bands, I have a symmetric fiber optic bandwidth of 100 megabytes, when I test my network from cable download at about 98 Mbps, but when I do it from the 2Ghz network downloads me at 20Mbps and when I do it from the 5Ghz it does it at 40, I think these values ​​are wrong, never configure anything of the wifi in terms of bandwidth .. could you guide me to increase my speed Wifi? thank you!

http://forum.mikrotik.com/t/wireless-download-mostly-very-slow/144659/1