[
From SQL Server Performance]
When we want to get the row count of a table, we use the transact-SQL function count (*). In the case of large table which may contain millions of rows, it can take a long time to return the row count of the whole table, and that leads to slow performance for the query.
The article continues at
http://www.sql-server-performance.com/articles/per/Speeding_up_the_Performance_of_Table_Counts_p1.aspx