Are you saying that the Jazzy wheelchair goes forward and backward at a good speed but when it turns left and right it slows down? If so, that is pretty normal. A power wheelchair will go slower while taking a turn when compared to going straight as a safety feature.
The Jazzy 614 HD is a heavy-duty unit that takes a lot of power to rotate. If you are noticing that it is behaving unusually, then perhaps you need to call a technician to take a look at the motor and controller.