As the customers and products table gets larger the Customer Reports (admin/store/reports/customers) gets slower and slower so that by the time you have 50,000 orders and 25,000 customers (my data set) the query never finishes. See this thread for discussion on the issue. As mentioned in the thread the slow part of the query is summing up the number of products in a user's combined orders. The fix, as it turns out, is easy: add an index to qty and the report becomes quite usable again. Patch is attached that does just that.