Outlook 2007 autocomplete is slow, can it be due to mailbox size?

A user complains about autocomplete being slow filling addresses in Outlook 2007.  I have taken her outlook.n2k file and put it on my PC and it works fine.  I reinstalled Office 2007 for her, still slow.  I gave her a different computer with a fresh install of Office 2007, still slow.  This is not the old nk2 file from Office 2003, it is relatively new and not that big ~700k.  her mailbox is huge, 1.6GB.  Could that have something to do with it?  I have disabled her add ins, indexing is not running, I have taken all the steps I could find on the internet to no avail.  Like I said the same nk2 file works great on my computer.  I even put my nk2 file on her brand new PC and it opens slow.  So can it be the size of the mailbox?
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I don't think the size of the mailbox is causing the problem.

To be clear, there are two versions of the "autocomplete".

1. If the email address has been sent to before in this install then it will pull from there.
2. If not, then it checks the contacts database for a match.
3. If multiple match is found, then it puts the red squiggly line under the name.

You might check your address book that it searches from and reprioritize. Get there by SHIFT+CONTROL+B and then go to TOOLS | options. See screenshot to see what you are looking for.

How much memory is in the computer? Check the memory usage in the Task Manager.

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