Choppy iphone videos on Mac

Hello mimibeeks,

Are these videos syncing via iCloud or are you transferring them in a different manner? Are the videos being played in the Mac's Photos app or in another one? To help get to the bottom of things, it may help to try using the Photos app if you aren't already, and then you may try exporting the video outside of the app to play it outside of the app: Export photos, videos, slideshows, and memories in Photos on Mac - Apple Support

1. In the Photos app  on your Mac, select videos you want to export.

2. Choose File > Export > Export [number] Videos.

3. Click the Movie Quality pop-up menu and choose the video format for the exported video.

4. Standard definition (480p) is good for standard-definition TV. If the video will play on a high-definition TV or display, choose one of the high-definition formats: 720p or 1080p. 4K is used for ultra-high-definition TVs and displays. 

5. Click the File Name pop-up menu and choose how to name the exported files (this step is optional).

6. You can name your exported files using the titles you’ve given them in Photos, their filenames, or a filename numbered sequentially. If you choose Sequential, you can specify text to appear before each number (for example, Birthday - 1, Birthday - 2, and so on). 

7. Click the Subfolder Format pop-up menu and choose how to divide the exported files into folders.

For example, if you export videos from several different days, you can have Photos export each day as a subfolder named after the day. Choose None to export the videos as individual files without subfolders.

8. Click Export.

9. Choose where to save the exported video files, then click Export.

Be sure to also check for any updates/upgrades for macOS to ensure it has the latest improvements available: Update macOS on Mac - Apple Support

Cheers.