For the third time in three weeks the villian in the piece has turned out to be Acrobat Reader. The first instance was very high cpu utilisation. The second was the incorrect configuration of associations for pdfs after an upgrade of Acrobat. The final was the slow opening of pdf documents.
The common factor was the same version of Acrobat reader. All of these incidents including version 9 of the reader. The answer was to upgrade to the latest release and this resolved the issue. So seems to be a potential update issue since the users claimed that they had updates recently applied.