To have a fast png/jpg export of the first page of a large document, the repagination should be limited to the page which has to be exported. (see https://bugs.freedesktop.org/show_bug.cgi?id=80879)
Confirmed that presently if you export to jpg or png, it will repaginate the complete document, though png and jpg exporting is limited to a single page.
It would e.g. prevent the export from having a *correct* value of page count fields. It would be a WONTFIX/INVALID IMO.