calling PdfImg macro for PDF files of many pages takes very long

[falkb]

A few minutes can go by until the Trac page is processed and displayed. As you cannot speed up the external tool 'convert' you should either warn about a long response time at runtime or limit the call of PdfImg to a maximum number of pages. Otherwise users are frighten off why displaying by PdfImg is that slow, although Acrobate Reader loads the same PDF file in 1 second.