Parallel execution improves the performance on the computers that have multiple physical or logical processors. To support parallel execution of different tasks in the package, SSIS uses two properties: MaxConcurrentExecutables and EngineThreads.
In my next article on SSIS Parallel processing I will cover how you can utilize the parallel processing capabilities of SSIS. In the later part of this article, I will provide some tips for SSIS Performance optimization and finally I will talk of what you need to take care of when executing your SSIS Package on 64-bit computers.
Refer this link for more details.http://www.sql-server-performance.com/articles/biz/SSIS_An_Inside_View_Part_4_p1.aspx