> is there any way we can parallelize the conversion process, or parts of it
Technically this is possible. You could parallelize conversion from multiple images to PDF (then merge files together) or to XOD then merge pages, h
however the question arises whether this is really necessary.
Once XOD is converted, you should probably cache it to avoid having to convert every time someone want to view the same document. You could also convert to XOD as soon as the file is added to doc library instead of at the moment user requests the file for viewing. Finally rather than waiting for the file to completely convert you can also on-the-fly convert and stream partially converted file to WebViewer (for example see WebViewerStreaming sample that comes as part of PDFNet SDK). This is probably preferable to parallelizing conversion because a user may want to close the document soon after it is opened. Kepp in mind that for best viewing experience (that allows to instant jumping to page regardless of doc size) we recommend accessing pre-converted XOD files and HTTPPartRetriever (which supports HTTP Byte Ranges).