The petrel is a nice design and should hold it's own at 4 mph and probably do O.K. at 4.5 (Nick would be better to respond to this than I would). Looking at the design I would guess that drag increases about 80-85% from 3.5 to 4.6 mph and probably about 100% from 4.6 to 5.8 (I converted from knots to keep it in the measurement you are using). That is what you are feeling. If you are mostly in calm water a boat with a little softer chine, a little less rocker and more tracking and a longer waterline length (no need for a longer boat length) would give you a slight advantage but you would be giving away some fun responsiveness which is what that boat is all about in my opinion. If you like the way the petrel paddles and responds I don't think I would make a change to pick up a speed increase of maybe .2 mph. If speed is what you are really after it is not the best design for that. With a boat designed for speed (not racing) and with similar stability characteristics you would probably only add .2 mph with the same effort. A boat designed for racing is designed for a much higher effort level and at speeds and effort level you are paddling at would likely have more drag and have much less stability. Speed does have a lot more to do with the paddler than the kayak.