Open Office very very slow

I have been running CALC via OpenOffice and LibreOffice for a few years. I do hit problems sometimes with one or other of them but usually they work all right.
I have tried to standardise on OpenOffice.
In the last few days I have been almost unable to open files.  The icon in the task bar sometimes get stuck in an incomplete state with just a blue line at the bottom, and a grey body. I have found sometimes that if I just leave it, it does open after 40 minutes or so. I cannot work with my files like this.
This happens whether the file is .ods or .xls.
I am on windows 10 and I did a system restore which set the system back to where it was before the lastest W10 update, and prior to a 7zip upgrade.
When I restarted the computer it immediately loaded the latest Windows 10 update and did not cure the problem.
I removed the profile file in OpenOffice and this was regenerated automatically when I reloaded.
I also reinstalled OpenOffice.
None of these actions made any change.
Please suggest what the cause might be.