Instructions
Launch "Flight Simulator X," and navigate to the main menu. Click the "Settings" option, and then click the "Customize" option to open the settings customization window.
Click the "Graphics" tab in the settings customization window. Click the marker on the "Target Frame Rate" slider and drag the marker to the "Unlimited" end of the slider. Click the "OK" button to save the settings change. Graphics settings will not take effect until you restart "Flight Simulator X".
Close "Flight Simulator X" by clicking the "X" button in the upper-right corner of the "Flight Simulator X" window.
Launch "Flight Simulator X" to affect the change made to the "Target Frame Rate" slider. "Flight Simulator X" will now display game play at the highest possible frame rate.