You get used to it. Words don't run into each other, you can recognize them as you do at slower speeds. Default speeds are too slow and inefficient for navigating around. You can easily go beyond default speeds, how high, that is a question. And, perhaps depends from person to person.
By flutter I mean, a kind of shiver in the voice or the voice getting more robotic than usual. In E-speak it occurs at 100% and at alll boost mode rates. It is still comprehensible, just annoying.