John wrote:

But why is his outlook slowing down after the 3 years of keeping email
in IMAP?
When it is caused by keeping all emails in that container - or a few
containers as Joe Mills wrote - what is the solution?

I'm not convinced that the size of the PST is the issue, but since IMAP PSTs
are ANSI format until Outlook 2007, I supposed it's possible. With IMAP and
even smaller stores, I experience performance issues. I don't think Outlook
is one of the better IMAP clients.
--
Brian Tillman