you shouldn't, it's optional, as stated in the
help file 
Quote: "It is not required that you call this command in your game loop, if you do not call it, Sync or Update will call it for you with the last frame time to allow the physics simulation to catch up in real time. If you do call StepPhysics then it will not be called for you by Sync or Update for that frame, this allows you to do your own physics timing if you prefer."
it also covers some advice on how to fix the "slow" down issue
Quote: "By using a fixed time step every frame your physics will perform exactly the same across all devices and all frame rates, but a reducuction in fps will result in the physics appearing to go slower, as it will always step the same amount of time whether the frame was quick or slow to draw. Using a variable step for each frame will keep physics moving at the same speed regardless of frame rate, but it will no longer be deterministic across devices and different frame rates. For example if your game depends on a physics entity falling and bouncing to the same height each time it is run then you should use a fixed time step. If the position of physics objects is not important to you game logic then a variable time step may be best."
life's one big game
spec= i5 4ghz, 16gb ram, Nvidia 1070ti gpu
