Hi Kevin,
Thanks for your inquiry. Document.PageCount property invokes page layout which builds the document in memory so note that with large documents this property can take time. However, after invoking this property first, any rendering operation e.g rendering to PDF or image will be instantaneous.
Since, rebuilding page layout is a time-consuming operation, if you call Document.PageCount property many times (e.g. every time after modifying the Document), it can slow down performance of your process.
Also, please note that memory and CPU usage are completely dependent on document size and document complexity. Upon, rebuilding page layout, on average, Aspose.Words layouts 10 pages per second. So, in your case, the extra amount of time Aspose.Words takes to rebuild page layout depends on the number of pages your Word documents have.
Best regards,