Today i was working with a SSIS package that has to load 22 million rows of data, it takes a Stored Procedure as source. The problems starts when you put the sp into the query window. SSIS immediately starts validating the columns, but he can only do it after loading the SP, which takes a while.
To disable this checking you can turn off ValidateExternalMetadata on the data source or DelayValidation on the data flow level. It now only checks once on run time not on design time. The downsite of using this is that the component is not aware of changes to the metadata of external data sources.
You can set DelayValidation to True on package elements whose configuration is not valid at design time to prevent validation errors. For example, you may have a Data Flow task that uses a destination table that does not exist until an Execute SQL task creates the table at run time. The DelayValidation property can be enabled at the package level, or at the level of the individual tasks and containers that the package includes. Normally you must leave this property set to True on the same package elements when you deploy the package, to prevent the same validation errors at run time.