The first time I click on the file, it comes up after about 30 seconds with the pop-up box - 'This file needs to be opened by the Excel Workbook text converter, which may pose a security risk if the file you are opening is a malicious file. I am using a Word Template file (.dotx) as a form with the links to the Excel file so that I open a new document each time I want to open it for a new case I have created it myself, all the data I have manually entered by keyboard myself, so there is no malicious software. The Excel file I am using is one I have used for years successfully with Word (previous versions) and have updated to. The Office version is 16.10 (180210) updated 15th Feb 2018 (latest update).

I am using MacOSX High Sierra version 10.13.2 (latest update) on a iMac 21.5' late 2015. Mail Merge between Word and Excel is going extremely slow and often opening a document where in the Mailings tab all the options are greyed out.