When did you last change your Wi-Fi password? If you haven't touched it since the installation, over the years that password has long since spread to the neighbors, to guests and to old devices. If your internet is slow, the reason may not be your line but uninvited devices sharing your network.
Changing the password is done from the router interface and takes five minutes. The only thing to watch out for is that after you change the password your own devices have to connect again with the new one — be ready for that.