Write Only Mode Speedup?

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Should write only mode be "faster"?

I had an issue with a normal workbook where writing a 20k row 200 column sheet is taking about 20 seconds. About 7 seconds to loop through my data and do the inserts (with ws.cell(xxx)), and about 7 seconds to call save_virtual_workbook to get ready to stream the result in Django.

I switched to write only mode. Looping through my data creation is now slower. It goes from 7 seconds to 13 seconds . save_virtual_workbook is now very fast, only .2 seconds.

But the overall time is basically unchanged. Is there something I am missing, or does it seem like write only mode just shifts performance slowness to a different part of my loop?