Another option, not a literal translation, but perhaps would translate what you mean (if I understood you well, of course).
Fazer isso deixa o arquivo mais pesado. To do that will make it (the file) take longer to load. (it means that it will make it too large and it´ll take forever to load it.)
Fazer isso deixa o arquivo mais leve. To do that will speed up Word/Excel/Writer/Calc (the program you are talking about at the time of speaking). To do that will make things work faster.
And one way to specify what is that "to do that": Break your file down into smaller files of 50-odd pages each. That ought to speed Word up. Then put it all back together when you've finished. (an example sentence from the Web)