How flat and horizontal you lie in the water. Good body position saves more energy than any improvement to your arm stroke.
Most beginners swim at an angle: head high, legs sinking, body ploughing rather than gliding. The cause is almost always head position — looking forward instead of down. Head and legs work like a seesaw: lift the head and the hips drop. The most effective fix is therefore to look diagonally down at the pool floor and keep the neck long, so the waterline sits around the middle of your forehead. Add light core tension so the hips don't sag. Only once your position is right is it worth working on the arm stroke — before that you're optimising the engine of a boat sitting sideways in the water.