Other reasons that your speed has slowed could be:
If you connect multiple devices to one wifi they share bandwidth, have you added any new devices to your network?
I would assume that your wifi is protected (has a password) if it is not then you could have some neighbors stealing wifi, and therefor bandwidth, from you.
Do you have a "guest mode" set up on your router? Guest mode is something that allows visitors to connect to your wifi without having to give them your normal wifi password. However some routers do it in a very unsecured way and if you have left it at default settings then other people "neighbors/strangers" could potentially connect to your wfi without you knowing. I would recommend turning that feature off!
Just...