Extract only the values for all Spreadsheet cells. These extracted values can be sent back to server (with minimum size) as server only needs data on each sheet. On other information required like references, formulas, images, colors etc.
On click of save or a separate button, which will generate a low size data file (having sheet name, cell index and value), which we can send to server.
It will save network traffic and also no complex parsing needed at server side to fetch data.