Perhaps I may have been using incorrect terms, so I'll use the terms that are used in Safari help.
I store my bookmarks in six folders.
These folders displayed in the Favourites Bar, that is below the Toolbar, and above the Tab Bar.
Along with the bookmarks stored in those six folders, there are also folders stored within these six folders, and they also contain bookmarks.
After opening one of the folders in the Favourites Bar, I get the beachballs whenever I hover over either a bookmark, or one of the folders within that folder.
Activity Monitor shows the Safari is hanging.
When Safari is beach balling, the computer locks up so bad that not even the dock comes up.
And as mentioned previously, EtreCheckPro on the three Macs, never shows a record of the Safari beachballs/hanging.
This problem arrived on the three Macs, as soon as Big Sur was installed on them.
Again, two of my three iPads that are running the latest OS, and have the exact same bookmarks, and bookmark folders as the three Macs, are operating great. The third iPad runs 10.3.3, but there are no problems with the bookmarks and/or bookmark folders.
This problem also happens with Safari Technology Preview, with the same bookmark folder setup.
It does not happen with any other browser (6) with the same bookmark folder setup, on the three Macs.
If I use the bookmarks shown in the Bookmarks Sidebar, there are no beachballs. The website icons for certain bookmarks that are shown in the sidebar, are the same as those shown for the bookmarks in the favourites bar. This leads me to believe that it's a problem with the favourites bar.
I now realise that having, 'Show website icons in tabs', unchecked, has nothing to do with the 'Favourites Bar' beachballs.
I've used OnyX to remove 'Bookmark icons'.
Are the terms, 'Website icon', 'Bookmark icon', and 'Favicon', interchangeable?
Whatever the correct term is, is there a way to prevent their formation?