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Finally seem to have found the another bottle neck.  In the calculation of the Pearson correlation each time getItemsNumber was called -- that calculated the total number of distict items by doing the full database scan (which was rather heavy).  Cached the number in the dataModel (since the number of items does not change).  Now the cross validation takes only 1,500 ms; comparing to original 150,000 ms (optimized a number of different things: neighborhood construction, correlation caching, etc.).  Thats a nice 100x speed up.  I am pretty happy about that.  So no need to optimieze this part of cross validation any longer .