Adam,
Try this -
Humans are smarter than chimpanzees, presently the smartest creatures on Earth. Does humanity represent the theoretical limit? Certainly, the human brain's hardware is far slower than the theoretical limit. Human neurons fire approximately 200 times per second, using signals that travel at a maximum of 100 meters per second. By comparison, my computer's CPU operates at 667 million clock cycles per second, and the speed of light is 300 million meters per second; the reason a human brain has around a hundred million times as much raw computing power as my computer is that a human brain has 40 billion neurons and 100 trillion synapses. If your neurons could be upgraded to fire 200 million times per second and send signals at 100 million meters per second, the result would be a millionfold "subjective speedup"; you could think a million times faster. In the time it now takes for your watch to count off 31 seconds, you could do a year's worth of thinking; more than a millennium of subjective time would pass between sunrise and sunset.
You might not be any "smarter" - you would simply think much, much faster - but the effect, to an external observer, would be beyond description. A community of ultraspeed humans could - mentally, at least - recreate the entire path from Socrates to World Wide Web in less than a day. A day after that, if the ultraspeed humans have physical technology that runs at the same speed as their minds, the ultraspeed community would have the same technology and culture we would reach in 4700 AD... and just 1900 AD to 2000 AD was enough to take us from steam engines to the Internet.
And that to my mind sounds like a change beyond imagination.
Cris