Hello, Erick,
Thank you for bringing this to our attention. After further testing, we have noticed that if the thumbnails are hidden, the experience in the loading time is improved. The following article shows what is the available public API for managing the visibility of the thumbnails menu:
https://docs.telerik.com/devtools/winforms/controls/pdfviewer/features/thumbnails
Please give it a try and see how it would work on your end.
Regards,
Dess | Tech Support Engineer, Principal
Progress Telerik
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