What type of router do you have?
Standard Consumer level wireless routers can only handle about 10
devices, after that you will need to get a better router, or another
router to extend your network on.
Also, if you are currently using a wireless router with the 802.11b
standard OR ANY DEVICE on your network does, every device will go that
slow. Not slow for the Internet, but for working within the LAN where
you may need things fairly instant, this will be very slow even if you
upgraded to 802.11g. Users requiring fast connections would best be
suited with a wired connection to the router.
Once you can handle speed on your LAN, you will also need to make sure
your Internet connection has enough bandwidth for your users. You
could try dowloading a large file from a fast server during a time
most people are using their devices and see how that speed compares
with the speed of the Internet Service you bought. There are many
speed tests you can run such as: http://www.speakeasy.net/speedtest/