I can't verify and confirm this with LO 4.3.1.2 (Win 8.1).

Could you please give more information about the different single steps to reproduce it.
Do you try to load a file directly from the Windows Explorer or from the last files in the LO Start Menu or through the LO menu FILE -> OPEN?  With which LO component do you experience this?  Can you reproduce this is also with a new file or do you experience this only with a specific file?

(In reply to comment #0)
> Problem description: Clicking on FILE causes a long dely before any of the
> dropdown menu appears, used to be an instant response before upgrading. 
> 
> Steps to reproduce:
> 1. Click on File
> 2. Long wait before dropdown menu appears
> 3. ....
> 
> Current behavior: As above
> 
> Expected behavior: Instant appearance of dropdown menu (eg to Print or Save
> etc.)
> 
>               
> Operating System: Windows XP
> Version: 4.2.6.2 release

(In reply to comment #0)
> Problem description: Clicking on FILE causes a long dely before any of the
> dropdown menu appears, used to be an instant response before upgrading. 
> 
> Steps to reproduce:
> 1. Click on File
> 2. Long wait before dropdown menu appears
> 3. ....
> 
> Current behavior: As above
> 
> Expected behavior: Instant appearance of dropdown menu (eg to Print or Save
> etc.)
> 
>               
> Operating System: Windows XP
> Version: 4.2.6.2 release

1. Version is 4.2.6.3, not as above.

The behaviour is the same on all files I have tried, both spreadsheets and text files, and also with new files. It makes no difference whether I open from Win explorer, from the recent files list, or from File>Open.

When I click on File (and this only happens with this command option, Edit, View etc. all work fine) there is a delay of very many seconds before the dropdown menu appears. When it does eventually appear it works normally. If during this delay period I then click on another option, say Edit, I get a windows message that the program has stopped working; eventually the message goes and the File Dropdown menu appears and works as expected.
I believe this is a LibreOffice problem, as I have tried several files in an older version of OpenOffice and all works normally, as indeed it did with LibreOffice before I upgraded.
I hope this helps to explain what is happening.

My LibO on Windows 7 behaves fine as well.
As none of us can confirm, I can't think of anything other than resetting the user profile: https://wiki.documentfoundation.org/User_Profile#Resolving_corruption_in_the_user_profile
..and also trying version 4.3.x.