A Microsoft file hosting and synchronization service.
This may have been repeated by others, but once you re-download the folders/files to your computer it will still seem a hair slow if you are using the "OneDrive" folder to open things. I've found that if you repoint all your shortcuts/favorites to the actual OneDrive.noindex folder that others have pointed out in Group Containers - OneDrive StandAlone the slow lag is gone.
Nothing like having this ultra fast new MacBook Pro M1 Max and feeling like its slow to open excel files...
This obviously is not much of a permanent workaround since the whole file syncing and keeping files on local drive options is unreliable. But if you want to work within your files faster I'd try that for now.