Use the Frame Rate field to set the frame rate per second for the output .swf file. The default frame rate is fifteen.

Flash movies typically use a frame rate of 12 frames per second. To achieve a similar frame rate, set MotionBuilder to 30 frames per second, and specify a step of 2 in the Step Time field. This forces the renderer to generate a frame at every second frame, resulting in a Flash movie with a frame rate of 15 frames per second.