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To resolve this issue, follow these steps:

1. Close Outlook.
2. Click Start, and then click Control Panel. 
3. In the view pane, click Classic View. 
4. Double-click Indexing Options. 
5. In the Indexing Options dialog box, click Advanced.
6. In the Advanced Options dialog box, click Rebuild.
7. Open Outlook.
8. On the Tools menu, point to Instant Search, and then click Indexing Status.
9. Verify that the number of items in the Indexing Status dialog box has increased.

Microsoft KB935400 blames the cause of the Outlook 2007 slowness to network hardware device, such as a router, that does not support TCP Window Scaling. However, the problem does not exist in Windows XP, thus the problem is probably more due to poor implementation of TCP Windows Scaling feature, or at least due to this feature that does not provide backward compatibility with older network equipments.

To solve the problem, simply disable the Vista AutoTuning function which is the most common suggestion and fix for various network problems in Vista.

Here’s the step to disable TCP Window Scaling Auto Turning feature:

Click Start button, click All Programs, and then click Accessories.
Right-click Command Prompt, and then click Run as Administrator.
If you are prompted for an administrator password or for confirmation, type the password, or click Continue.

At the command prompt, type the following command, and then press ENTER:

netsh interface tcp set global autotuninglevel=disabled

Try to download the emails with Outlook 2007 again. The slow downloading problem should have resolved. Microsoft also suggest users to isolate the hardware that does not support TCP Window Scaling, replace the hardware and re-enable the autoTuningLevel parameter.