When you get a wireless router from your ISP you set it up and if it works, all is good, But most routers work on the same default channel and if you live in a terrace or a flat then there's a good chance that you'll suffer interference from your neighbours, which can cause the slowdown.
Changing the channel is the easiest fix for the problem, while other factors that would hamper services included the router's position compared with other radio equipment in the home.
There's a lot of competition on those frequencies, Cordless phones, TV remote controls and microwave ovens can all cause interference.
Changing channel and repositioning the router away from other wireless devices can make a big difference.