A damaged walking belt, improper belt tension, lack of belt lubricant or a bad drive motor can cause the walking belt to slow down when you step on the belt.
Unplug the treadmill, then check the walking belt for damage and replace the belt if it's torn.
If the belt is in good condition, check belt tension by lifting the middle edge of each side of the belt. If the belt is properly tensioned, you should be able to lift the belt's edge up 2-3 inches above the walking board. Adjust belt tension if the belt is too loose or too tight.
If belt tension is okay, then you might need to add belt lubricant to the walking board so the belt slides easily across the board.
A weak drive motor can cause the belt to slow down when you step on the treadmill. If the belt is properly adjusted and lubricated, then you might need to replace the drive motor if the motor can't spin the roller to drive the belt at the set speed.