Microsoft Office Customization Installer - Very Slow to Start

Question

I've developed a fairly simple VSO Add-in for Excel using Visual Studio 2019 on a fully Windows Updated Windows 10 box.

I use the Build > Publish menu to publish the add-in to a mapped network drive (my Z: drive) as "From a CD..."

Everything seems to work OK.

However when I switch to a different machine (also a fully Windows Updated Windows10) and run the setup file it takes a very long time for the [ Install | Don't Install ] form to come up.
By "very time time" I'm talking an hour or more.
The machine I'm installing on is a powerful laptop with little resource usage and a very good network (& internet) connection
It doesn't matter if I run it straight from the file-share, or copy it locally to run.
I've tried signing the code so I don't have a certificate error/warning and that did not help.

Help?

I've no idea where to look so even start to work out what is happening here!
This is not a problem with the add-in itself -- when it finally installed one-time it load into Excel and ran (printing out some debugging messages)