Slow speeds on WiFi are commonly two factors.
1) The wireless card being used on the endpoint devices. Not all WiFi cards are built the same. Sure, you think 802.11n means that you should be able to get 300mbps/450mbps on a device, but it could be a energy savings WiFi card where the max link speed is 72mbps.
2) The "channel busy" state of your access point to your wireless devices. You may have devices inside your home that's causing busy states on your WiFi because of interference. Cordless phones, other wireless access points broadcasting in the area, standard EMI can cause channel busy states where you will get less speeds to your laptops and mobile devices.