Many users are complaining about slow WIFI or dropped connections using their newly updated, or purchased, iOS 8 devices. I am also experiencing this issue. The fix for me was pretty simple. Open Settings > Privacy > Location Services > System Services (scroll down to the bottom) and turn off WIFI Networking. This does not disable WIFI or Location Services. This settings uses WIFI connection in your area, connected or not, to augment location services.
Hopefully Apple will produce a fix quickly, but I personally don’t this is a bad setting to leave off anyway.