I remember working in an office environment with some very large powerpoint files (50-100 slides with large images), and the rule of thumb was that 'openoffice is free, but MS can handle the big stuff'. Don't know if that's still true, haven't tried OO in a while.
OpenOffice is basically dead in terms of development nowadays. If you're looking for a desktop office suite on Linux, LibreOffice (a fork of OpenOffice) is still actively maintained.
I tried Libreoffice writer briefly recently. I imported some large pictures onto a single page and it became unusably slow. It seemed to be redoing an expensive computation (likely the pictures) with every character I typed. So yes, clearly not as optimised as Word. I had to reluctantly ask to borrow my wife's computer with MS Office for this particular task.
