The plan speeds they have quoted are theoretical, and in some markets, simply not acheivable at all times for reasons well above our heads.
To that end, wireless speeds in your home will be reduced to the least common denominator, and will slow for each additional client that is connecting wirelessly. To maximize your network's efficiency, you should use hard-wired connections for as many things as possible.
The other thing you may want to investigate is interference. If there are many others in your 'hood using Wifi, there is likely interference. This is common in most suburbs and certainly all cities. If you see 20 or so wireless networks when you are trying to connect to your router, then it is certain your performance will suffer as a result of interference.
Perform a speed test from a WIRED client directly to the MODEM (bypass the router) to determine if the problem is in your setup or out in the world.
Copy and paste the modems signal level and error log pages and the results of a speedtest using the TWC tool on a wired computer.