I'm not sure what the spec of your router is but a lot of home routers can only handle a few devices connected at one time, so 7 would normally be alot for the router, more so if you are gamers.
If you are all using wireless please let me make you aware (if you are not already) that wireless gets slower as more people join the router. The router can only send or receive at one time(some can do both) so the more people connect the longer you have to wait until the router will send data to you again. Wireless is not like cable where you can send and receive at the same time and all devices can talk at the same time.
Try putting in a cable direct next time it is slow and see if it is any better?