why locks a table will improve performance during transaction?
Let's say you have a million row table.
One million individual row locks is a lot of memory that can be put to better use.

I'd also read some SQL Server articles on lock escalation:


SQL Server Storage engine team http://blogs.msdn.com/b/sqlserverstorageengine/archive/2006/05/17/lock-escalation.aspx (selected quote, my bold)



  Similarly, if you lock individual rows, you will get higher concurrency but then you will incur the overhead of acquiring/releasing locks on each row and lot more locking resources depending upon the isolation level of your transaction, as you may need to hold the locks on all the rows till the end of transaction.



DBA.SE: https://dba.stackexchange.com/a/6449/630
DBA.SE: https://dba.stackexchange.com/a/6515/630