imported iMovie video is not smooth

i have recorded a video with my iphone 6 and began working on it through iMovie on my iphone.

However, during my editing of the video, i realized that the imported video is not as smooth as the original video taken from the phone. when i saved the finished product and compared it to the original video, the two videos had clear differences in terms of smoothness.

it seemed as if the imported video in iMovie had less frames than the original one. Is there anyway to solve this problem?

thank you.