the fps is very low when the other window on the top of game window

#1 I know that Torque2D uses a preference variable : $Pref::backgroundSleepTime

By default set to 200, it means that when focus isn't on the game window, processing occurs every 200 ticks. Setting it to a value of 0 makes the game run at full speed even when under other windows, etc. Pretty sure T3D uses a similar (if not the exact same) feature.