Because of overhead in the Wifi standard, best case you're going to get 45 - 50% of the rate its showing you're connected at. Throw in interference which is pretty common on the 2.4 GHz band and its not that surprising.
You also have to remember that in order to adhere to the N standard, an AP has to be polite. If it can hear a neighbour's AP anywhere on the 40 MHz chunk your AP is trying to use its going to fallback to a single 20 MHz channel. Remember that there is only 60 MHz of spectrum available in the 2.4 GHz band so there is a good chance of this happening.
So your best bet is using 5 GHz radios that support 2+ streams but even then the range on them is lower than 2.4 GHz radios. You may need an extra AP or 2 to get the level of coverage you want depending on the size and layout of your place.
I regularly get 130+ Mbps over 5 GHz with my 2 stream radios.