Paste to Table in Word Very Slow

I am using the latest version of Office 365 on a Dell desktop with huge storage.

I have a document consisting of a large table (60 pages) - it is an index. When cutting small bits of text (typically 5 or 10 characters) from another document and pasting them into a table cell, there is a delay of about 20 seconds per paste. The problem does seem to relate to the size of the document (although it is only 360K) because when I temporarily delete most of the table the delay disappears. However, I have older indexes created in earlier versions of Word that, when opened in Word365, do not exhibit the same delay.

I have been through all sorts of suggested fixed found on the web but none makes any difference whatever. I have also restarted Windows, run repair on Office365, saved the document as Word 97, cleared temp files etc but to no avail,

Any suggestions would be welcomed