The increasing reponse times seems to be a bug with N1QL DP4, @geraldss will know more about that.
Using Observables from the Java SDK is going to help you if you want to analyze the rows that get returned in “real time” and then perform operations on those. It won’t help you with the current issue because it seems to be a DP4 bug (the same behaviour can be seen when using the .NET SDK as @jmorris identified).
In your case you are just printing stuff, what do you eventually want to achieve? We can then see if an asynchronous solution will give you better performance.