Extreme speed uses a bursting feature to optimize the balance between speed and system responsiveness. Since bursts are usually several hundreds or thousands of trials at a time, this feature affects a number of features like precision control, confidence testing, and checking for cell errors.
Crystal Ball tests for precision control (and performs confidence testing in OptQuest) after each burst of several hundred or thousands of trials. In Normal speed, these checks are performed every 50 trials by default. As a result, you may see the simulations run more trials than usual when these features are turned on. Generally speaking, these features are not as critical to simulation performance in Extreme speed as they are in Normal speed.
Because of bursting, Crystal Ball cannot check cell errors after every trial as it does in Normal speed, only at the end of a burst. If cell errors occur in forecast cells (such as "Divide by zero" or "#VALUE") and the Run Preferences, then Stop On Calculation Errors option is selected, the simulation stops at the end of the next burst.