There are many reasons you could be having issues with your Airport.
One of the easiest to fix is:
- you are nearby other wifi networks using up a lot of similar network channels (very likely if in a large apartment complex)
- you have devices in your household which are using a lot of similar or conflicting bands (think portable phones, especially base stations, universal RF remotes, etc.)
With any of the above situations, you will want to tweak your router's channels to make sure you get optimum reception. Of course, distance matters a lot - the closer you are to it, the clearer your signal will be, and the faster it will therefore be. Your devices also should be supporting the same advanced wireless protocols - the newer they all are, the better your range & speed will be.
Also, you will want to make sure you are positioning the router properly. It should not be against a wall or near metal, or any other wireless devices. It goes without saying that you shouldn't have anything on top of or below the router, as that can also significantly interfere with the signals being sent out.
You may just want to review an article like this to make sure you are setting everything up properly: http://www.shilling.id.au/2014/05/05/airport-extreme-review/
Finally, if nothing helps, and you are getting awful speeds from wireless on all of your devices while sitting next to the router, you may need to replace the unit. They do fail sometimes, but usually more spectacularly and just stop working instead of giving you slow speeds...
another article on speed/channels/signal strength:http://www.macworld.com/article/2014318/evaluating-the-strength-of-your-airport-network.html