Roundtrip latency is certainly going to add up to a substantial chunk of time, but that's not much excuse to discard performance considerations. Requests that take say 50ms of processing will take even longer if the box is busy; it doesn't take much to add up and becomes noticeable.
And if your implementation on the server side is very fast, you can do more.
My point was indeed that performance can matter, and will matter even more in the future, since no apparent technological limit on network speed has been reached so far.
And if your implementation on the server side is very fast, you can do more.