We have been dealing with poor performance with Excel 2003 after the High Priority update 2501584 was installed on client computers. This update adds the MS Office File Validation that originally shows up in Office 2010 into Office 2007 and Office 2003. For most users with Office 2007 they have not seen any issue in performance after the update was installed. For many with Office 2003 they see huge hits in performance when opening excel files across the network even if it is just the 100MB local LAN.  With one client they were experiencing 15-20 delays in opening the file with the update installed. After uninstalling it that dropped down to just 2 seconds to open the file up. Today when looking for the update to remove from the computer and see about hiding it from installing I found this “fix” from Microsoft on the issue (http://support.microsoft.com/kb/2570623).  There is a Fix it button and installation which did the fix for you or you can follow the steps below (copied from the site) to do it manually.  These steps either manually or through the Microsoft FixIt application keep the file validation checking on for all programs except excel which seems to be the only one that is experiencing these performance degradations.

Let me fix it myself

You can use the EnableOnLoad registry entry to configure how you want
Excel to handle opening workbooks for the OFV. By default, the
EnableOnLoad entry is not present in the Windows registry. To add the
EnableOnLoad entry to the Windows registry, follow these steps:

1. Exit Excel.
2. Click Start, click Run, type regedit, and then click
   OK.
3. Locate and then click to select the following registry key:

   HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice11.0

4. After you select the key that is specified in step 3, point to New on
   the Edit menu, and then click Key.
5. Type Excel, and then press ENTER.
6. Select Excel, point to New on the Edit menu, and then
   click Key.
7. Type Security, and then press ENTER.
8. Select Security, point to New on the Edit menu, and
   then click Key.
9. Type FileValidation, and then press ENTER.
10. Select FileValidation, point to New on the Edit menu,
    and then click DWORD Value.
11. Type EnableOnLoad, and then press ENTER.
    Note: The default value
    is 0 which disables the validation.
12. On the File menu, click Exit to quit Registry
    Editor.