PDF Viewer very slow

We are opening a 12 page pdf document, with some images and text on each page. The pdf viewer is very slow to open the document, perhaps 12 seconds And the viewer is slow to navigate to the next page. It is even slower when thumbnails are displayed. For reference, the same document opens in Edge, Chrome, Acrobat, in less than 1 second. Any hints on how to speed if up ??

It seems the PDF file contains some heavy images. Please be advised that .NET is not as fast as the native apps. We are working on improving the performance of this one further.