If your clothes dryer takes a long time to dry, or overheats and turns itself off, you may have too much lint in the exhaust duct. It is sometimes possible to locate special duct brushes for clothes dryer ducts, but you can make your own with a kitchen cleaning brush. Locate the type that has nylon bristles coming out of a looped metal holder. Twist the metal to fit into the ventilation duct -- and you have a custom sized ventilation duct brush as shown in the photo.
If cold air comes back down that exhaust duct, you may want to try a better exhaust hood on the outside, that closes firmly. A commonly available one is a large white tube that attaches to the house and bends upward, with a plastic cap floating over the exhaust hole, all enclosed in a larger casing. A bit large, but very efficient. It never backdrafts, never freezes shut and no birds can find their way in.