It seems to be a pattern with (not just) Microsoft products to hang the UI while waiting for some remote API, usually talking to the home base. Happens a lot to me with Office desktop apps - they freeze sometimes for a good second or so while starting and more surprisingly also when closing. One time it bothered me so much, that I recorded an ETW and unsurprisingly the time during the hang was spent waiting for some http request. I noticed that often the online version of these apps (namely Outlook) are more responsive than the desktop counterparts.
Edit: My experience with Excel - https://twitter.com/martin_ky/status/983019737729916930?s=20