We've been developing ribbon apps that help users manipulate documents that can range from 2 pages to 3000+ pages. What has consistently killed us is that when we update the documents using the Word API, word must 'repaginate' for massively extended periods
of time. Even worse, we have to forcefully clear the undo buffer or Word will hang for minutes at a time after we've updated the document in our software.
We make extensive use of OpenOfficeXml to manipulate these documents and it would be wonderful if instead of hanging and repaginating, the word developers would figure out how to keep this from happening.
As a side note, the OpenOfficeXml documentation is available in both PDF and Word format. If you open the document that describes the OpenOfficeXml content, Word is essentially unusable.
If you think I'm kidding, download the ECMA-376 1st Edition part 4 from