Continually-runing resync... A 'chat' with support highlighted that my problem seemed to be a sync that was stuck - the systray icon showed no info but the progress info in the GUI showed 1,688 files being synced. Then that'd change to 1687 then back to 1688 then 1677 then 1688, occasionally a few more then back to 1687/1688.
I also noticed ffrom 'Process Hacker' (a sort of super-Task Manager) app that the DB client application had apparently read 1.8 TB of data since starting some hours ago... but there's only 130 GB of files in my DB. I don't know how much data you'd expect a reindex to read, but as hardly any of my files had changed since shutting down the machine last night and the reboot this morning after which the reindex etc started, I thought that that increasing read total, and the resync file count not decreasing meant that DB was perhaps reading and trying to sync one file over and over again. The #files count would go up occasionally when something else in my DB got saved (eg my notes on what I was doing) then come back to the same sticking point.
I'd also noticed (through a monitoring process I have here) that I'd gained a "conflicted" file (a notes file that had been updated shortly after I rebooted this morning). It's the first "conflicted" file I've had in several years, a symptom of the reindex/resync problem rather than a conflict between two machines accessing the same file.
Anyway, I shutdown the DB client, then manually sorted out the diffs between the conflicted file and the genuine one, then restarted the DB client. And the high cpu use etc has gone away. The client now seems to be behaving again.
