After we have further confirmation this solves the issue and these steps are fine-tuned, then we could publish this as a public solution. For now, I'll suggest an order to try things. I'll keep my original wall of text at the bottom of this document to preserve details.


1.) Press the Windows key and search for Default Apps. Scroll down and click Set defaults by app. Ensure Word, Excel, and other Office apps are configured as default apps for their associated file types.


Test and see if this fixes the issue. If not, we can eliminate this step.


2.) Open Firefox. In upper right corner click the three bars for Menu > Options and scroll down to Applications. Ensure the correct Office apps are set to open their respective file types. If Firefox has never opened these files before, it may be necessary to open them from SharePoint in order to be prompted for a default app.


Test and see if this fixes the issue. If not, we can eliminate this step.


3.) Completely uninstall Office 365 using the uninstall support tool found by scrolling down to “Option 2” on this page:


https://support.office.com/en-us/article/Uninstall-Office-from-a-PC-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8


Restart when prompted.


4.) Go to office.com and re-install Office 365 from the Install menu in the upper right corner.


After installation completes, it might be necessary to quit and relaunch open web browsers.




I suspect completely uninstalling Office, restarting, and reinstalling Office will eliminate the need for steps 1 and 2, but I've included them for now since they are easy to test quickly.




Here's the original wall of text that should not be included in the future public version of this solution:


Yesterday SharePoint was very slow to open documents for Theresa. Amy walked by and said she had been having the same experience intermittently, and that she couldn’t attribute it to network congestion these days since hardly anybody is on campus.

In Chrome on Theresa’s computer, when clicking an excel document in SharePoint, before opening the file a message would appear saying EXCEL.EXE is downloading.

I went to office.com and installed the Office apps. I waited for the install to finish and restarted the computer.

When opening a word or excel file in SharePoint from Microsoft Edge, I was prompted to choose a program to open the file. Word and Excel were not options to choose. The only available option was to choose an app from the Microsoft store. In the MS Store, when clicking on Word for 1 user, a promotional Microsoft site opened in a web browser.

I reinstalled Office again without restarting.

I installed Firefox, opened the same files from SharePoint, and associated the file types with Word and Excel.

At this point Office files began opening from SharePoint without issues from any browser.

 

I looked at extensions in chrome and deleted one that had already been disabled and indicated “this extension contains malware”

I downloaded MalwareBytes and ran a scan that came back clean. After the scan I uninstalled MWB to avoid popups after trial expiration.

I checked the Windows App Defaults and they were already set correctly.

 

If this slowness persists, the next step I’ll try is completely uninstalling Office using the uninstall support tool found under “Option 2” on this page:

https://support.office.com/en-us/article/Uninstall-Office-from-a-PC-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8