It is me, the OP. To simplify the story. - start TB. OK - fetch POP3 mail. A few times in a few hours. All OK. - do a "Compact Folders". messages say it completed. The blue activity line remains wavy(*). TB is idle yet it shows 60-70% CPU in "top". If I minimize TB then it stops using CPU. If I select a folder (but do nothing) the %CPU goes up to around 120%. Turning off the status bar also stops using CPU!
This last item probably points at the source of the problem!
TIA
(*) the item at the right side of the bottom status line shows a rolling blue/white pattern, as if it is active.