This is simply the way preview is due to image processing. It drops video frames to try to keep real time and the audio more continuous. You need to export it if you want to see it play with full frame rate. You can use the following to improve speed of preview:
If you still see the problem when you export, then Shotcut has a problem reading your video. It does not support variable frame rate video, and most mobile phones capture with variable frame rate. This is not something that will be accepted as a bug. Video editors cannot simply work with every possible file even though Shotcut tries very hard to. In that case, you need to use Properties > Convert to convert the video into a format that is edit-friendly.