When performing a Custom Space Export to PDF for a specific subset of pages, both contentToBeExported and contentToBeExcluded parameters are used for content calculation.
This is usually not a problem for smaller spaces, but can cause performance problems for large spaces (i.e 20k+ pages), because if you only pick 2k pages to be exported, flyingpdf will use contentToBeExported to calculate the 2k pages, but would also use contentToBeExcluded parameter to calculate and list 18k pages not to be exported, making the whole process significantly longer and more process intensive, than it needs to be.
contentToBeExcluded parameter is not used by PDF export plugin because the resulting content tree is created by filtering with the contentToBeExported alone, so it should be excluded from calculations.