That seems to rule out the time problem.
Is all this stuff in a single PST file?
Have you checked that X1 is paying due attention to all of the folders in Outlook?
My main Mailbox PST is 3 GB. In all I have 10 PSTs with a total size of 20 GB.
These are not all connected to my main mail profile. I attach older PSTs to an archive profile, with no related email accounts.