Ping. It's a signal that is sent out and it comes back. It's like sonar, but it tells how fast it goes out to the server and back. That then determines your speed on the network.
That alone won't help; check your network log via your computer and see how many devices are connected throughout the day and evening. If there are more devices than you have connected to the network at anytime, then you have a wifi thief.
I know exactly what it is, but he's sounds like a noobie so I'm trying to clarify what the problem is because "speed" isn't too clear. DL "speed" could be slow. Edit: Just read the title >.<
