You can speed up the sim but you can't slow it down unless the frame rate is much too low. The keyboard short cut control + t will increase the time speed. You'll need to press those keys 5 times to totally cycle through all the options. So if you already have 2x enabled (for example--it may say something else if it can't actually get that fast), you should try using ctrl + t 4 more times to turn it off entirely.