Because it is a raster image processor, it rasterizes pages at 1200 DPI CMYK raw files, then applies proprietary color correction profiles, algorithms, imposing booklet and so on.

I tried a totally blank page, if even 1 mm bigger than 13*19 inch (330*489 mm) the server crashes and reboots (I presume OOM)

So when there are one billion vector objects or transparencies, it takes like 1 minute per page