Yes, it takes longer than using the original field names. However, sometimes is required, since your database names may not be helpful to the further development (I'm thinking of SAP).
I use to do all the necessary steps when loading from the data souce and store the results into QVD files (even Excel spreadsheets), then create another load from the QVD, that shouldn't require any additional work and the load will be fast. It's a two tier scenario but it allows you yo work faster based only in QVDs in the development time.
I didn't mean keeping the optmized load from QVDs. If we are talking about that, then the QVD will load optimized even renaming and using one WHERE EXISTS() clause. Further than that, the optimization breaks and the load time will depend on the hardware, background running software, memory and so on.
And all the above obviously depends (and it could be viceversa) on the volume of data you are going to load. If performance is not critical because of the number of records, aggregations, joins and hardware, you probably don't find much difference one way than the other.
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