Using LO 6.2.8 on macOS 10.14.6, following a copy+paste operation onto a lengthy Writer document, the spinning ball of doom appeared and persisted across restarts of the app and computer.
Checking in Activity Monitor revealed the CPU usage by LO was spiking repeatedly >90%.
I tried restarting in Safe Mode and downgrading and then upgrading LO without effect (because of the current blurry fonts issue with LO, I had not upgraded to LO 6.3).
User Lupp on this forum post (Slow typing and high CPU usage after a certain point.) revealed the problem and solution – the copy+paste had inadvertently copied a non-printing character from the pdf that I was quoting “”. Viewing the paragraph text in TextFixer (https://www.textfixer.com/tools/remove-line-breaks.php) revealed the offending character and removing it appears to have solved the problem.