Go to export space, uncheck some pages from the export, choose to export. Confluence proceeds to copy all attachments from the whole space into the temporary folder used to create the export, even from pages which weren't chosen to be exported.

This causes the export to be unnecessarily resource-intensive in spaces where some pages contain multiple large file attachments which are unnecessary for the export. For example, I just exported a rather large PDF file which took quite a long time to export - checking what might be causing this I noticed the temp folder contained about 300MB of PDF files which were from pages that weren't exported and wouldn't used in PDF exports anyway.