Reporting from Big Sur 11.4: Same issue. I tried following in the terminal to kill the process % kill 15 com.apple.quicklook.ThumbnailsAgent % kill <the_processid_for_com.apple.quicklook.ThumbnailsAgent> % sudo killall com.apple.quicklook.ThumbnailsAgent But it kept running. Then I “force-killed" it from the Activity Monitor. No heat so far.