Preview video lags/stutters at certain points

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Shot on iphone 2 BH6.mlt_ - Shotcut 2020-08-07 10-47-13.mp4 (video)

You have proxy on, but you did not generate proxies for the clips. If they have proxies it would say “(PROXY)” under their names in the timeline.

Proxy Editing

Introduction Proxy editing is the process of creating and using low resolution videos and images in places of the original or optimized (Convert to Edit-friendly) files. This provides the advantage that your computer has less work to do in realtime while editing: decoding, scaling, and effects. This is, quite simply, because there are less pixels to compute. Then, when you are ready to export, it will use the original (or optimized) files for full quality. Performance is optimal when the pre…

Also, I do not see the problem you describe in your videos. It is playing quite smoothly. If you expect it to be smoother, it will not be inside the editor. You need to export to view it more smoothly. If that does not work good enough for you look elsewhere.