At a previous job, I debugged a close relative of that exact problem: sorting list views was very slow when you wanted to sort on a column with very few distinct values in a large table. The culprit was of course a quick sort degenerating to n squared performance.
Had this problem in xBase (Clipper with NTX indexes): sorting by date but there were many null (empty) dates in that particular column. Added another date column (that was never null) to the index just to provide entropy.
