It so happens that if you live in a large house and your wireless router is in one corner of the building. Then there might be a dead zone in the opposite corner of the house where the wifi signal is unable to reach. Remember, anything that interferes with wifi radio waves produces a dead zone. If the house you live in was built before wifi was innovated, then the floorplan and building materials might interfere with your wifi. This is due to the fact that old houses may have thick plaster walls and the metal wiring tends to interfere with the wifi and block it. In addition are the large metal objects such as filing cabinets or metal walls that also block a wifi connection. Besides these reasons, is the interference of competing networks which create slow wifi zones. Yes, competing networks tend to cause slow connectivity in an area due to the same or overlapping channel. Note that when a wifi channel has a lot of active users on several networks, that time the speed decreases for every user.