There could be many reasons as to why you are experieincing low download speed such as inteferecnces caused by other devices like wireless phones, microwave ovens and other electronic devices operating on the same frequency. Also applications running in the background that depend on the Internet can also impact your speed as well. Sometimes, certain types of games do that, It is important to check this as well. Here are some tips that might help you improve your speed.
1. Use WPA2/AES and enable WMM - most 802.11n Wi-Fi certified devices will slow down to 54 Mbps if you use WEP or WPA/TKIP security, since the 802.11n specs state that the higher throughput rates can't be enabled if either of those outdated security methods are being used. Similarly, the 802.11n spec requires devices to support 802.11e (QoS enhancements for wireless LAN) in order to use high throughput link rates, i.e. higher than 54 Mbps. WMM is a subset of 802.11e that is required for products to be certified for 802.11n.
2. If you are using channel bonding only use it for strong signals - many newer 802.11n routers support channel bonding, i.e. using 40MHz instead of the default 20MHz channels. This only works for strong signals and small distances. At longer ranges, channel bonding can actually reduce your performance by 70%+ ! Simple throughput testing using both channel widths should be performed for your specific location, or simply turn channel bonding off to accomodate weaker signals.
3. Keep routers clear of interference - as mentioned above, most routers/modems/gateways are not well shielded against electro-magnetic inferference, and they'll work better when away from any possible source of EMI/RFI. Keep your router at least one foot away from any other routers, modems, switches, computers, monitors, power supplies, fans, fluorescent lights, cordless phones, etc.
4. Use uncongested wireless channels - to further reduce interference, be mindful of other wireless networks in range that may be competing on the same frequency. Many current Wi-Fi routers are pre-configured to use the same wireless channel (6), and it can be very congested if you are in an environment with other networks. Do a wireless survey of your area and use a channel at least 3 channels away from those used by other networks in range to avoid interference.
5. Improve signal strength - it is best if wireless access points are at an elevated point, at least one foot away from any surface that may reflect or attenuate a significant portion of the signal, such as metal/foil insulation, wire-mesh stucco on exterior walls, etc. If installing near the edge of a building, there may be an advantage to using a directional antenna, or building a simple reflector using aluminum foil behind omnidirectional antennas to focus the signal. This is less of an issue with some newer 802.11ac wireless routers capable of beamforming with built-in phased arrays. If your router/ap uses removable external antennas, another easy option to improve signal is simply replacing them with longer/stronger omnidirectional antennas. *Note: The underlined piece of information is only an idea and you do not have do that unless you deem it necessary.