There's no generic "megapixel per second" speed for PDF-rendering - just to make that clear.
On the iMac you're running Geekbench 4, where the creator of Geekbench defined his own artificial benchmark for PDFs (this is the very nature of benchmark programs, not just Geekbench). Using that program you get a MP/sec speed, but it is not comparable with any other MP/sec speed rating except those obtained using the exact same method as Geekbench.
The method used by Geekbench is to parse and render a specific 29-page PDF test document using the PDFium library.
On the iPad, you're normally using Apple's own PDF rendering libary. As it is not PDFium, you cannot compare with Geekbench.