How increase video speed into iOS without change video FPS?

Question

I made a video with 30FPS and 18s. I want increase x3 the video speed and get a new video with 6s (18s/3).

I did the resolution bellow:

let newDuration = CMTimeMultiplyByFloat64(asset.duration, 1/3)
compositionVideoTrack.scaleTimeRange(CMTimeRangeMake(kCMTimeZero, asset.duration)
                                        toDuration: newDuration)

The resolution is creating a new composition and scale the CMTimeRange. But the video result have x3 FPS too, the result video have 90FPS.

I want a video result that has 30FPS and 6s, is it possible?