I recently upgraded to Office 2007 and generally my experience in terms of performance was negative. It runs absolutely slow! The worst culprit of the lot was Outlook 2007.

Googling the issue thankfully worked and I located this brilliant site by Roundtrip Solutions with enough tips to resolve the problem. The tips listed are a long read and for me all I had to do was:

Shane Keller has offered the following general solution on the Microsoft Outlook Discussion forum, which has been refined through feedback by other users as they implement it:

STEP 1: Turn off Outlook 2007 and locate each of these files in turn and rename as indicated. For some reason, during the upgrade these four files can become corrupted - the migration from 2003 to 2007 isn’t the smoothest but not having to create all your POP3 accounts again is excellent.

1. extend.dat - you will find this file in the location | Drive Letter:\Documents and Settings\user-directory\Local Settings\Application Data\Microsoft\Outlook\extend.dat

//change it to extend.old

Long live Shane Keller!