I had the same problem with Outlook 2003 becoming too slow to use.
I think I've narrowed the problem down to disk access.
It was taking several seconds to switch folers, and deleting an email could take a second or two also. I moved the PST file onto a RAM disk and now Outlook is humming.
It takes a lot of RAM (you'll need as much ram as the size of your PST file), and it is dangerous becuase if you have a power failure or crash, you'll loose any changes since the last backup, but if you live on Outlook the change can make you much more productive.
