How do I change the password on the router as has been suggested?
Everyone should do this anyway if they never have. It's just good security. 
You'll need to log on to the router itself. In your browser address bar, try 192.168.0.1 - if that doesn't work, try .0 or .2 at the end, instead of .1. This may work, but it may not, depending on the IP addresses used. You will then be asked for the password. If you've never set it before then it may be on a sticker on the back of the router, or underneath, and the password listed there will work. If not, try "admin", "password" or "changeme". The username will probably be admin.
If you've got this far, then you will probably have some sort of webpage menu, and somewhere will be an option to change the password, which could be under a heading like Security or something.
If you change your password, make sure it's something you remember. 
This may not work anyway, but it's worth a try.
Also, check whether your ISP has support pages which explain how to configure your router (assuming it's an ISP-provided one.) That would probably have pictures and a clearer explanation of what I'm trying to say.
If you've got a teenager who does online gaming, or someone's watching TV online or something, the network performance will probably go back to being bad once you've given them the new password so they can connect.