why locks a table will improve performance during transaction? I just started to read about table lock in SQL (from this page)

It saids:


  table lock may reduce the overhead of acquiring a large number of row
  or page locks and save overall locking time


From my understanding it will prevent other query to access the table I have locked, but why will this improve the performance? Isn't it will slow down over roll queries when there is mutiple user trying to use the table?  Can someone explain little bit more?
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
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