WiFi download speed

I recently upgraded my internet package & modem to the newest Cox provides (Arris 2460) and installed by a Cox serviceman. using a LAN connection I connect at or close to the 300 Mbs, I realize using a wifi connection you can expect up to a 40% loss in speed. sitting next to my modem connected to the 5ghz network I can only achieve a maximum of 40Mbs. download speed. upload speeds are usually higher or equal to my download speeds. this is considerably more than a 40% loss. what can be done to remedy this problem? I'm considering using another ISP if i see my connection buffering again. please help