slow mouse = timid gesture = small type
fast mouse = bold gesture = large type

This works in brushwork if the fast hand is also a pressing hand - that is, in some ways this is a substitute for a pressure-sensitive tablet interface. On the other hand, I wonder about wiring the algorith the opposite way, as if it were a drippy watercolor brush:

slow gesture = spreading drip = large type
fast gesture = light smear = small type

I find the equivalency between speed and font size more interesting than the kind of equivalency, I suppose. I haven’t played with Illustrator for a while, but don’t most vector graphics suites with pressure tablet support allow you to do something like this kind of “font brush”? If this is a web-reimplementation of just one such feature, I suppose this would go towards Lev Manovich’s arguments that software editing suites are the great art of our time.

Of course, there is something compelling about any feature of a complex system when separated and considered in isolation….