My wi-fi was slow and I couldn't figure out why. I downloaded one of those programs off the internet because I suspected others were using my wi-fi in the neighborhood. I had given my wi-fi password to my older neighbor to use for one week. I think he passed it along to others in the neighborhood. I installed the program and changed my password. It also had a feature on it that would tell me when others would try to log onto my wi-fi. For the following few days the beeper went off over and over with other's trying and trying to get on my wi-fi. This lasted about a week and then tapered down to now none. Here's a link to Kim Komando's info on how to secure your network and info about the wireless network watcher program.
Check to see if your neighbors are stealing your Internet | One Page | Komando.com