So to confirm, straight upgrade to 6.12.4 had major GUI delays, so restored 6.12.3 great for about 2 weeks then same problem started to occur in 6.12.3, I uninstalled connect and no help, so reinstalled. Then I uninstalled all 3 unassigned devices plugins, and GUI issue gone straight away. So then upgraded to 6.12.4, No GUI issue, for a few days so installed unassigned devices plugins (3) now, noticed a little delay in loading the main tab for the first time each browser restart but only 1-2 seconds, and after it is loaded the first time revisits are instant, all other tabs working as before, fast. So will update if anything changes.