Disable "Hey Cortana" feature is not enough for me. I need to disable Cortana at all.

GREAT! It solves the issue for me too. Well done filip. Now I hope for I fix from Microsoft, I like Cortana, it's funny. Meanwhile I'll disable it.

The same here, just tried. It askes me to close edge but after is the same.

I don't think so. I have no modified host file.

Maybe related to online profile... onedrive integration? Who knows...

Ok, but before reverting back, did you try to open an excel file?

Reading old solutions for slow excel, before windows 10, seems like it was related to the Intranet traffic setting inside Internet Explorer.
I searched for the same settings inside edge options...

I don't think you can disable it.

Now I'm @work, my PC has Windows 7 so I can't try. You can make those quick tests:

1) Reboot PC, don't open edge browser and create an empty excel file on the desktop (right-click, new excel...

Other question: Do you have any adobe (creative cloud) app installed?

I'll try too tonight.
One question. Which Antivirus do you have?

It happens to me too, on all 4 PCs which was upgraded (including a Surface Pro), all with Office 2013. It's an issue of Excel only, Word for example works very well. Obviously is more noticeable on...