....but you said in your first post ....
I've tried doing a clean up of the iMAC, repairing the OS, running it with all extensions disabled in Chrome and running without the AV, none of which have improved things.
..... so it WAS the AV ?
Good spot and I really can't remember exactly what I did when I 'stopped' the AV previously.
This was a belt and braces uninstall and reboot, which I have done twice now with a reasonably repeatable outcome, once a week ago and again today.
Having played about with the AV components, it looked like the firewall component the first time I reinstalled ESET, but this time it seems to be the Web Access Protection. so the jury is still out on which ESET component is the actual root cause.
I'm sure I have a valid Kaspersky licence as well, so may just see if that works any better; although Kaspersky for Mac didn't used to be that good.