It looks like you are using UIA 2.0 (default on XP and possibly Vista). If possible, can you try upgrading to UIA 3.0 ?
The problem is that UIA 2.0 does not support virtualization, so automatic peers are created for each item in the completion lists (large enough), and the algorithm that does this does not scale very well. UIA 3.0 supports virtualization, so only the list items that are visible are created by automatic peers.
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