V5 Motors not running at constant speed

Recently I put 4 V5 motors on a simple and light weight base with each motor directly connected to a wheel. When I push the joysticks forward the robot moves forward with what seems to be the full motor speed, but then decreases in speed right after the initial burst. I am wondering if this is normal for V5 and if so if I could change in the code to keep the motors running at full speed all of the time so that it doesn’t de accelerate?