In GWT, why shouldn't a method return an interface?

Question

In this video from Google IO 2009, the presenter very quickly says that signatures of methods should return concrete types instead of interfaces.

From what I heard in th

Answer 1

This and other performance tips were presented at Google IO 2011 - High-performance GWT.

At about the 7 min point the speak addresses 'RPC Type Explosion':

For some reason I thought the GWT compiler would optimize it away again but it appears I was mistaken.