Just a few thoughts on this. What kind of speed are you expecting? What are you using to test your network speed? Are all your switches GIG switches?
You might have to pull a wall jack and look at the cable, you can usually yank out enough to see the cable, it is usually marked every foot or so. Even if it is cat 6 , you might have had a crappy install causing your issue or you have some hardware issue bottle necking your network. Can't help you much without more info.