Recently I have seen Microsoft Office 2003 taking an age to open Excel documents over the network. You can use Wireshark to see what is happening at the network level:

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You can see that it is opening the file a teeny tiny piece at a time. This can cause quite a delay as the spreadsheet gets larger. It turns out that it had been caused by an update to Excel called ‘Microsoft Office File Validation Add-in’:

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It can also appear as KB 2501584. You can remove this add-in and the problem goes away. You can find more information here. There are also details on that link which are easier to roll out to a large number of users, either the MSI or the registry change. The long term fix is to upgrade to Office 2007, apparently. Er…