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.
emir82
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