There could be many problems for this...I've forgotten what worked for me, sorry.
If you go to the MS Outlook 2003 Support Centre click here;EN-GB;out2003 and type slow in the Search box you get 25 problems/solutions listed. One might fit your specific problem.
If none solve your problem you can go back to the Support Centre (or click here;EN-GB;KBHOWTO&Product=out2003), do a more advanced search & ask for more solutions to be displayed per search.
Hope you solve it - I know just how frustrating this problem can be : (