Thank your for the request. By default, when you open a web-page with GroupDocs.Viewer widget, and the target document is invoked (for displaying), on the server-side GroupDocs.Viewer converts the document to the images and text (for the image-based rendering) or to the HTML, images, fonts, and CSS (for the HTML-based rendering). Only when this conversion process is finished, GroupDocs.Viewer begins to transmit data to the client-side and end-user can see content of the document in the browser. That’s why it takes so much time - GroupDocs.Viewer needs to convert all document, and they are really big. The conversion speed depends on the performance of the server - CPU, memory, storage.
However, there is a “PreloadPagesCount” method. When using it, for example, “.PreloadPagesCount(1)”, GroupDocs.Viewer begins to transmit data to the client-side when only first page of the document is converted. After applying this method to the 1st document (17 MiB), GroupDocs.Viewer shows its first page almost instantly. All other pages are shown on demand, when you try to open them.
There are some other methods to increase performance:
1. Use “PreloadPagesCount(1)” method. When using this method GroupDocs.Viewer will begin to load document to the client-side when first page of the document will be converted. When this method is not used, GroupDocs.Viewer converts all pages of a document and only then sends it to the client-side.
2. Use “MinimumImageWidth(value)” method. MinimumImageWidth: if set, the Viewer will load page images with the specified width from the server when starting. It will not load page images from the server again after zooming if the current page image size is smaller than the value specified and if the original (not scaled) page image size is smaller than the value specified. It means that in most cases the GroupDocs.Viewer will load page images only once and will not reload them after zooming in/out.
3. If this is possible for your business-logic, disable thumbnails (“ShowThumbnails(false)”).
4. If this is possible for your business-logic, disable ability to select text by using “SupportTextSelection(false)” method.
5. Use “ShowViewerStyleControl(false)” method - it will disable “double page flip” option and will increase performance.