SharePoint 2010 Grouped List Slow for Drop-Down Filter Menus

Hello,

I have a SharePoint 2010 List that uses two layers of grouping. The list loads relatively fast, in a second or two from when requested. There are about 5,000 items in the list. I have left the default pagination in.

When a user goes to select a drop-down on the header and select "Show Filter Choices," it takes forever, 30-60 seconds, for the drop-down filters to appear above each column. This is needed, as the data being filtered is not intuitive, so any other filter web part such as a search feature would fail to be useful. We want users to be able to filter directly from the list, presented with the option of all possible values - which the "Show Filter Choices" does.

What is odd is that I've noticed that if I remove the "Group By" criteria from the list, the "Show Filter Choices" load instantaneously. There is no delay.

So, the massive delay is a combination of "Group By" + "Show Filter Choices." This is a SharePoint 2010 instance.

This looks to me to be closely related, if not identical to: http://social.technet.microsoft.com/Forums/en-US/155897ba-a55c-4a1f-9adc-2e935433fe1d/grouped-document-library-view-drop-down-filter-very-slownot-working, for which no good answer was ever found.

Any idea on how to solve this? I tried adding indices, to no avail.

Thanks,
Joseph Irvine