You probably need better wifi but this can also be a problem with your internet's capabilities. It all depends on how much bandwidth your network has and how many devices are connecting at any given time. For instance, say your wifi offers a total of 10 Mbs and is split between 6 computers and n laptops then you'll be splitting 10 Mbs by 6 and n. Do you get what I'm saying? Likewise if it's your internet connection although connections nowadays are usually (at least) 100 Mbs so that shouldn't be an issue.
^^Hopefully I got the above right^^
Basically, check your specs for your wifi as well as you internet speed and get back to us. Worst case scenario you call your local ISP to come down and configure your network for you.
A common cause of WiFi network slow-downs is due to using the default signal channel. Example: on 802.11b networks, there are 13 channels. The default that all WiFi routers (access points) use is channel 6. Other AP's that your systems can see are likely using channel 6 as well. So, go into the configuration tool for the router/access-point and change the channel to something other than 6. I have found that in many situations this helps tremendously, especially in an environment where there are other AP's in visible operation - common in commercial (business) buildings.