Say you want to show the total sales from a list that has 1,000,000 or even 10,000 items. To do this client side, you'd need to request all 1,000,000 items and calculate the total on the client machine. Using a DVWP, the calculation happens on the WFE, so the data never has to travel over the network. (Technically it has to travel from SQL Server to the WFE, but that's going to be a very fast connection.)
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