In my experience PDFJs has excellent performance. I find it consistently less laggy and more responsive than Orion's builtin PDF viewer.
You can investigate performance with the example viewer here.
According to the FAQ, performance is more dependent on file complexity/size than the number of pages. Not exactly sure why this is 🤷
Not only is PDFJs more feature rich, I also find it more robust. It avoids some bizarre freezes I've had with Orion's builtin pdf viewer. Not sure if Safari has these issues with its pdf viewer. Its been so long since I used that.
The PDF viewer is one of the reasons I still keep Firefox as a secondary browser. When I'm working with a lot of pdfs, I'll start moving tabs from Orion to Firefox just to take advantage of Firefox's better viewer.
As a bonus, switching to PdfJS would also resolve the find text in PDF feature.
NOTE: I've tried using the PDFJS Chrome Extension for this, but unfortunately Kagi doesn't seem to support it (doesn't process the URL correctly).