Thanks for trying other camera apps, Rick.
Yes, I tried many of my other camera apps also... Pro Shot Camera, Office Lens, ... can't remember them all now because I deleted them one by one to test if there were any conflicts of apps.
Only Lumia Camera 5 and Coach's Eye were the only apps I found to instigate the slow 4 second delayed auto-rotate; and yes, everything else is affected once the problem arises. Only a restart of the phone seems to restore normal rotate functionality again.
So, to think an update to Lumia Camera 5 to fix it may be hopeful, considering that at least one other app using the Denim OTA update also exhibits the same behaviour.
I don't really want to do a factory reset, you lose all pictures and videos (and saved games) that aren't backed up in "the cloud", right? Last time I did something like this, was upgrading from 925 to 930, and some of my purchased music weren't available anymore.
Still early days, hopefully there's some form of update of app/firmware that might resolve this.