Suggest you also vacuum out the line from the dryer to the outside vent.
Just went through this. Did not want to pay big price for duct cleaning since it is a second story laundry room and vents to the roof. Got the contraption called Lint Eater at Lowes and hooked it to the shop vac and got it all cleaned out in no time. Highly recommend the Lint Eater. Vids on You Tube show how easy it is.
We have done that also. Thanks for the suggestion! The length of the line is the primary source of backpressure.