Slow Opening Office Files

I had a user who said that it was taking 3 to 5 minutes to open a file from OneDrive for Business or SharePoint with both Word and Excel. Opening local files was fine.

I had him untick the "Use Office applications to sync Office files that I open" box in his OneDrive settings and now he says files open normally. 

However, that is not really a fix for the problem. Most, if not all, of our users have that box ticked and have no issues so the real problem must be something else. 

Any ideas?