How to fix slow wireless network speed ?

Below is a list of tips to understanding your wireless network issues and improving wireless speed.

Have realistic expectations - first, please have realistic expectations for wireless network performance. The theoretical maximum speeds for 802.11 Wi-Fi variants quote ideal conditions, transfers in both directions and shared between all network devices. Real-life actual downstream speeds considering protocol overhead are in the 30-40% range of theoretical maximum speeds for wireless devices, you can find more information -here-. Also, please note that in data transfers, speed is measured in Megabits per second (Mbps), not Megabytes. For reference, see our bits/bytes conversion calculator.

Use 802.11ac or 802.11n clients - note that a newer 802.11ac router will not improve your speed if you only use 802.11g/b clients. Also, many 802.11n network adapters are rated at a maximum link rate of 150 Mbits/sec. Don't mix clients - your brand new simultaneous dual-band 802.11n router will slow down by 50% to 80% to accomodate any legacy 802.11b/g clients (while they're activelly transmitting or receiving).

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WPA/TKIP security, since the 802.11n specs state that the higher throughput rates can't be enabled if either of those outdated security methods are being used.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.

Only use channel bonding 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.

Keep routers clear of interference - 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.

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.

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.

Tweak advanced router settings In addition to choosing an uncongested channel/frequency, WPA2-AES and enabling WMM, you can further improve the quality of your wireless network by tweaking some advanced settings, often only found in custom/open-source firmware, such as dd-wrt. If you have already flashed dd-wrt on your router, take a look at the following:
Sensitivity Range (ACK Timing) - reducing this from the default 2000m to 100-300m will provide a cleaner wireless environment by dropping packets from other weak wireless networks that may be in the area.
TX Power - bumping up transmit power (slightly) may improve signal. Be careful not to overcompensate, as increasing this a lot will stress/overheat your router and may put it outside the allowed FCC transmit power. It also amplifies noise together with signal.

For more in-depth information, you may also want to read our related articles:
Network Adapter Optimization
Wireless Network Speed Tweaks