1. Try the Office Repair function.

2. Uninstall & re-install Publisher.

Tried office repair made no difference. Even installed Publisher 2010 and it's behaving the same way. All other apps, including Office apps, are running as smoothly as they always have. It's a mystery. Seen old posts on many forums with people experiencing the same problem but no solution.

The page in the link below might offer some additional things to try. Some solutions appear to work for some users with similar issues regarding speed:

Publisher 2016 running VERY SLOW

Well, for some reason everything is working fine again. All of the settings appear to be set to their defaults so I'm still clueless as to what happened. All running nice and smooth again, even images dragging perfectly (no legacy drag). My PC was on all night and I don't see any updates but I'm happy it's all back to normal. Just bugs me that if it happens again I'll be none the wiser! Thanks for your replies folks.

I forgot to suggest uninstalling or disabling any add-ons or plug-ins you might be using.If you can't remember all the settings easily, I recommend recording them all now while Pub is working well, so you'll have a comparison tool if it glitches again. Either screenshots or phone photos will do it quickly.

Great idea! I have taken and saved screenshots of all the settings.