Complex Spreadsheet, very high disk usage working out of SharePoint directly

[Myles Gallagher]

I'm trying to figure out what's the root of this issue,  but the gist is I have a very formula heavy document that is currently misbehaving dramatically.  Co-authoring has gotten very flaky, not too surprised about this, the doc is slow, also expected.  What is perhaps most interesting, I am working directly through the SharePoint document library  instead of through OneDrive synced folders, and when attempting to "save" some large changes excel starts spinning at around ~30% CPU of a quad core, and constantly at 30-50MB/s disk activity!  This went on for well over 10 minutes then ended and the status changed from "Saving" to "Saved".  I've no clue why there is so much disk activity for a file that is not even stored on my hard drive!  I assume excel is saving some temp files or something, this is expected but the file is not even 3 MB in size! 

What could excel possibly be doing that amounts to ~18GB of disk activity!!!

[#2]

I've no idea. But here's a couple of things to check:
- Download and run FINDLINK by Bill Manville (google for that, you'll find it easily) and run it to expose all hidden external links in the file. Get rid of the ones you know are faulty
- Check the Styles list (Home tab, gallery in the Styles section). If it contains a lot of them (check the same gallery when you open an empty workbook to compare), you should remove the obsolete ones
- On the home tab, click the magnifyer and choose "Selection pane". Then go through all worksheets to check if there are any unexpected objects on the sheets.