You could change the encryption to WPA-PSK and see if you notice any changes. To see if anyone else is using your connection click `Configure Livebox` and select Security - look for Attached Devices or similar (I used one just the other day, I can't remember the exact terms used) match the 3 numbers with your currently connected hardware. If there are more than 3, there's your answer. The Livebox is pretty safe in that it only accepts new device connections for 10 minutes after the button on the side is pressed when setting up.
Have you done any speedtests? click here
Keep a log of speeds etc with something like NetMeter