possible causes.
explorer addon's (many apps add things like right click menu's or other additions such as google drive e.g.)
network drives
malware
slow attached storage
to check for addon's you could fire up something like process explorer, and right click the executable to see what's running on it. Also autoruns program will give you info in the explorer tab.
Thanks for the interest and suggestions.
I have previously run Win task manager, displaying on a second VDU while the delay was active but can't see any process struggling or not responding.
I'am convinced the issue is somehow network related as previously reported, running in Win safe mode without network support completely cures, in safe mode with network support the issue is still active.
Remember I have the same issue on all 3 of my Win 10 PC's with different Win 10 Home revisions, the only common factor I can think of is the LAN connection.
I run the latest Bitdefender on 2 of the PC's plus Malwarebytes Premium, just Win Defender on the other, all fully up to date, don't think I have a malware or virus issue.
I do have a WD NAS on Gb. LAN connection, however, the delays experienced are writing to internal SSD's in all cases.