For more information on this, take a look at this article:
Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6
If Internet access is slow on your computer, confirm your network supports the correct Wi-Fi standards. 802.11n provides the fastest Wi-Fi speeds. Other standards are fast, but you should confirm that your computer is using the fastest protocol available
Be aware that other computers or devices connected to your network may also contribute to poor performance. If you are not sure if a computer associated with your Wi-Fi network is causing poor performance, try turning it off or disconnecting it from the network.
Also this can always cause a bunch of issues.
AirPort and Bluetooth: Potential sources of interference
Have a nice day,
SOLUTION FOUND to MacBook Pro poor Wi-Fi wireless signal.
First, let's talk about the actual problem: too much signal noise. There is a hidden app in OSX called "Wi-Fi Diagnostics" This will tell you everything you need to know why your Wi-Fi is not working!
HINT: It is included in 10.7, just look for it.
Turn off your wi-fi and run your internet through your home's electrical wires. Power-line adapters are devices that turn a home's electrical wiring into network cables for a computer network.
I purchased TP-LINK TL-PA511 and for the first time I'm downloading with ALL my internet speed (30mb down / 5mb up) tested via www.speedtest.net. I'M SOOOOO HAPPY!!!!