I just took mine out for a whirl loaded up. Rode 70 miles up and down steep hills all day (westchester county, NY. The picture is a beach in CT) At first it was heavy, but no more than panniers were. After riding with it for a while I noticed that I forgot about it and just grew accustomed to the weight. It's easy to forget about after a while.
Most of your effort will be pulling the weight... not dealing with drag which is probably so subtle, it's not even worth worrying yourself over. IN FACT, I find that a trailer helps keep your momentum going on flats, as you get some "feedback push" from the trailer every pedal stroke, which also allows for long coasting. If you keep a good momentum going, the trailer kind of pushes you along and keeps the pace quick and steady.
My experiences were with a load of about 45 pounds.