jan_opo provided the answer!
discussions.apple.com/thread/250148717 mds_stores high cpu usage because of “extensions”-preferences
I watched the cpu% jumping up for the process mds_stores, always when I was on facebook. Accidentally I found a solution! Just fyi, unchecking “facebook” in the sharing-menu on the “Extensions” panel in system preferences kicked the related cpu usage for the process mds_stores back to zero! Didn’t try it yet, but obviously it could also affect other checkboxes on the “Extensions”-panel, like twitter, flickr etc. in conjunct with the related browser-activity.