Editor Stutters on slightly larger files or more overlays

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You need to use preview scaling and proxy editing if you want to improve performance while editing.

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, you should be aware that when you drop a new file into the timeline, it needs to compute audio levels for the waveform, and that will take longer on longer files. Finally, the display of the waveforms in the timeline slows things, and you can turn that off temporarily in the timeline menu.