"internal" = memory to memory = very very fast, mircoseconds. Blocking reduces the number of times the client must handshake with the server over the network. As I recall it's two or three messages each before and after each data block - regardless of the actual number of rows in that block. Time for each would be in milliseconds. As always, it depends on the speed of the network , traffic level, and load on the server, but with a substantial result set the difference is often discernible to carbon-based life forms.
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