The Performance Penalty
When you open a folder in macOS Finder containing hundreds of large video files, high-resolution RAW photographs, or complex PDF documents, the operating system attempts to generate a tiny visual preview (thumbnail) for every single file. While this helps you identify files quickly without opening them, the process of extracting these previews is highly CPU-intensive. If you are working on an older Mac or accessing files over a slow network drive, Finder might automatically generate thumbnails for minutes on end, causing the entire system to lag, the fans to spin up loudly, and battery life to plummet.
Relying on Generic Icons
To prioritize system performance and stop macOS Finder from automatically generating resource-heavy thumbnails for your files, you must configure the View Options to use standard, generic file icons instead.
Open the Finder application and navigate to the specific folder where you want to disable previews (or you can apply this globally). With the Finder window active, go to the top menu bar, click View, and select Show View Options (or press Command + J). In the floating panel that appears, look for the checkbox labelled Show icon preview. Click this checkbox to remove the checkmark. Immediately, all files in that folder will revert to generic icons (e.g., a standard blue ‘MP4’ icon instead of a frame from the video). To apply this performance-saving setting to every folder on your Mac, click the Use as Defaults button at the very bottom of the View Options panel.