When printing a report from the Web Report Viewer the report is rendered completely in PDF and then this pdf is streamed to the client browser. In case the report shows large amount of data and has lots of groupings and aggregates, it is expected its processing and rendering to take longer. Moreover, streaming the larger PDF file will slow the print operation too.
With the latest version of Telerik Reporting Q3 2007 we have introduced the capability to configure the rendering extensions. As far as the PDF export is concerned you can choose whether to embed the fonts in the result document or not. Embedding the font will increase significantly the size of the output document. For more information please see Configuring Telerik Reporting
the Telerik team