Why Rotation Is Instant But Deletion Changes the Document Structure
Rotating a PDF page sets a display attribute — a page-level rotation value (0, 90, 180, or 270 degrees) that viewers respect when rendering — without touching the underlying content stream at all, which is exactly why rotation is instantaneous even on large files and never affects text selectability, searchability, or image quality (see Complete Guide to PDF Processing for the deeper structural explanation).
Deleting a page is a fundamentally different operation: it actually removes that page's object from the document's page tree, and depending on how much content is shared between pages (embedded fonts, images referenced by multiple pages), a deletion can meaningfully change the resulting file size — unlike rotation, which has essentially zero effect on file size regardless of how many pages get rotated.
Combining both operations in one pass (rotating some pages while deleting others, all before a single re-export) avoids doing multiple separate round trips through the document, which matters more the larger and more sensitive the source file is — fewer separate operations means fewer distinct moments the tool needs to process the full document.