No offence to anyone, but I would not consider the performance of MS office to be great.
I guess it's comparative, but then I compare to its previous editions which used a sliver of the resources to accomplish 95% of what modern o365 does.
Well of course, but given that the previously employed technique was blitting, fliedumping memory areas, you can't be more efficient than that. Using XML is for transparency (readability).
(And, note, in context, I regard the ms office file format as lousy. The OpenOffice/LibreOffice format is good.)
EDIT: Narrowed the date.