Recently I ran into an issue where editing a Word document was painfully slow. Every time I typed a letter it was followed by an unbearable lag. Typing a sentence would sputter in dribs and drabs. This was not going to work.
The document in question was stored in SharePoint Online. I was using the Sync feature for easy access from my local computer.
The fix was actually quite simple.
Fix Slow Document Editing
Check the bottom-right of your Word document. If you see something that says Updates Available click it. If successful the Updates Available notice will disappear and Word will perform at its normal speed.
If you hover over Updates Available before you click it, you will see a description of what is actually going on.
It informs us that, “This document has been updated by another author. Click to save and refresh this document”.
This will happen if someone else has worked on the document since you opened it. When you click Updates Available it saves the document to the server and refreshes your copy with changes the other authors have made. If both authors have been editing the same content a conflict will occur. In that case, when you click Updates Available you will be asked to select which edit to keep.
For a great Microsoft article on simultaneous editing check this link.