Possible slowness issues when you upgraded from Office 2003 to Office 2007:
Office 2007 (Upgraded from Office 2003)
Several files are not properly converted during the upgrade
Search for and delete these files if they exist:
outcmd.dat, extend.dat, views.dat and frmcache.dat. Outlook will recreate these files as needed.
*.pst files = create a new pst file (Outlook 2007 format) and migrate your data into the new pst file, then delete the old.
*.ost files = Completely remove this file and let Outlook recreate.
Remove any un-needed Add-ins (Tools, Trust Center, Add-Ins, Select Comm Add-ins from the drop-down at the bottom of the window and hit GO)
Create a NEW profile and migrate data into it if you are using a pst.